ONE AND A half million people stroll among the redwoods of Muir Woods every year. Which might be why you haven't been to see them recently.

Muir Woods ranks just behind Alcatraz on the Bay Area tourist hit list, making it one of the most popular national parks in the country. But what local wants his or her redwood reverie interrupted by thousands of others trying to experience the same thing? The solution: visit during the evening, when the area, officially closed to the public, feels less like a national monument and more like a forest. Naturalist Michael Ellis is your ticket in.

Back in the '80s, when Ellis lived in nearby Muir Beach, he'd ride his bike to the woods and sneak in after dark. Now a hometown celeb to the crunchy set who listen to his natural-world perspectives on KQED-FM, he periodically gathers the fleece- and Gore-Tex-clad troops for fully Park Service-sanctioned after-hours Muir Woods jaunts. On this late April evening around seven o'clock, with the sky still light and the last trickles of day-trippers heading to their rentals in the overflow parking lot, our hike is just beginning.

Before the 20-odd night walkers in our group – all locals, with the exception of two Londoners – have left the parking lot, Ellis has made a wildlife sighting: a banana slug, which he gamely licks to prove its banana-ness and gross us out. The banana slug is a hermaphrodite, he tells us, having adapted to a sluggish life slowly chewing through forest debris, where one is unlikely to meet many sexual partners. Carrying both sex organs, the creature has a 100 percent chance of being able to mate with any other banana slugs it meets.

Armed with that knowledge, we enter the forest, and the coast redwoods – the tallest species of tree in the world! – take over. They're the main draw in this living museum, which preserves the few redwoods close to San Francisco that weren't logged to rebuild after the 1906 earthquake and fire. (Hence the name Mill Valley.) But even their staggering solemnity fades into the oncoming darkness, amid the sounds and movements of Muir Woods wildlife.

A plucky winter wren performs a five-second song and is identified by Ellis as a male jockeying for territory, asserting himself as night falls. He'll start singing again first thing in the morning to tell the other winter wrens he's still there.

As we pass along the banks of Redwood Creek – 21 other creeks in California carry the same name – Ellis points out the neat, mossy stones that line the banks, part of a failed improvement effort during the '30s. The vagaries of the creek had caused some cherished trees to tumble down, and resource managers decided to tidy things up. But while straightening out the waterway and removing debris made the creek swifter and more orderly, it eliminated the standing pools and riffles essential to the steelhead trout and coho salmon that spawn here.

Thankfully, those crick-fixin' days are long past: a flashlight beam on the water reveals the tiny fry that have recently hatched here and will eventually make their way to the Pacific as full-fledged, ocean-ready fish.

Night's coming on, and we ascend, leaving the paved pathways and wooden walkways for actual dirt. A bridge is out up ahead, so Fern Creek Trail is closed. But we're treated to the hoot of a female great-horned owl as we pursue our alternate route. Along the well-trod path of the old railway entrance to Muir Woods, our flashlights lend only the narrowest perspective.

There's barely time for the growing darkness to take on a sinister cast, though. From the Alice Eastwood site just above Muir Woods proper, the Saturday-night campers' acoustic classic rock remains audible as we reach the hike's climax, above the redwoods, among the low scrub of the chaparral. Even this close to the light pollution of San Francisco, Jupiter, Saturn, Venus, and Mars are visible to the naked eye, especially with a guide to tell you where to look. And with Ellis's high-powered spotting scope, the rings of Saturn come through clearly. Hold your own binoculars very still, and you may be able to pick out the moons of Jupiter too.

As we descend in darkness, our flashlight beams catch two young black-tailed deer grazing, the wriggle of a yellow-spotted millipede on the path. The millipede emits an amaretto-scented cloud of defensive cyanide gas when Ellis picks it up.

Yes, the gift shop is long closed, but that whiff of the millipede's almond-flavored defense beats any souvenir.

If you go

Muir Woods can be visited at night only with a group.

Michael Ellis leads his next excursion June 5. $30. For more information about this and other trips, go to www.footlooseforays.com.

The National Park Service offers ranger-led full-moon walks in Muir Woods. Free. Next program: June 2, 7-9 p.m. www.nps.gov/muwo/programs/index.htm.