The Monterey Bay Aquarium in California houses some of the finest marine exhibits in the world. So when the staff recently offered me a personal, behind-the-scenes tour, I couldn't refuse. Tim Cooke and Ed Seidel made the visit absolutely fascinating, and I am indebted to them for their hospitality. Tim, an aquarist extraordinaire, even let me in on a few secrets for raising plankton.

And he should know them: the Monterey Bay Aquarium grows a lot of plankton. Tim rears tons of the stuff each year to feed the thousands of voracious fish, crustaceans and jellies under its care. But these single-celled critters are not just fish food: they are quite intriguing in their own right and can provide amateur scientists with endless hours of delightful observation. When viewed under a microscope, the tiny phytoplankton (plants) and zooplankton (animals) are amazingly beautiful and complex.

These creatures can also be useful for many kinds of research. For example, phytoplankton such as green algae are great for investigating the fundamental biochemistry of photosynthesis. And members of a zooplankton group called rotifers, which measure a mere 400 microns across, serve as the microscopic equivalent of a miner's canary, because they are sensitive to toxins and therefore may be used to monitor the health of estuaries and streams.

Amateurs can easily rear both marine and freshwater plankton for examination, for feeding larger aquatic animals or for use in more advanced research projects. Ocean enthusiasts should go to their local aquarium store and purchase a kit to make 50 gallons of seawater (for about $15) as well as a simple salinity tester. You'll need to order the plankton from Aquaculture Supply or call 352-567-0226. Make sure they also sell you a copy of Plankton Culture Manual, by Frank H. Hoff and Terry W. Snell (florida Aqua Farms, 1999; $26.50)¿the bible of plankton cultivation. I recently grew up a batch of Nannochloropsis (catalogue no. AA-NCP, $8.50) and Tetraselmis (AA-TET, $11), both green algae that can live in either freshwater or salt water. And I raised a little saltwater rotifer known as Brachionus plicatilis (AB-R1S, $10). You may also want to grow diatoms¿a type of algae that strengthens its cell walls with fantastically beautiful silica structures. If so, a good choice might be Chaetoceros(AA-CHA, $11).

Clear plastic soda bottles in the two-liter size make ideal culture flasks. To prevent yours from being taken over by bacteria, you'll need to sterilize everything before you begin. So go to a store that sells pool supplies and purchase granular chlorine. Dissolve as much of the solid as possible into 30 milliliters (about an ounce) of warm water. Then prepare a 10-to-1 dilution by mixing five milliliters (one teaspoon) of the concentrated chlorine solution into 45 milliliters of distilled water. Be careful you don't transfer any undissolved crystals into the sterilizing solution you are preparing.

Next, fill your two-liter containers nearly to the top with either distilled water or seawater and add five drops of the sterilizing solution to each. Wait two hours for the chlorine to do its work. Chlorine evaporates quickly from solution, so you'll have to make up a fresh batch of sterilizing fluid every time you need some. In this sense, evaporation is a nuisance, but you can take advantage of it to remove the chlorine in the flasks by bubbling air through the water for about 24 hours. A few drops of bottled dechlorinating agent from a tropical-fish store will do the job in seconds. Either way, don't introduce your plankton until you've verified, using a kit for testing home pools, that no chlorine is detectable.

A single pump for a 10-gallon aquarium can easily aerate 10 culture flasks. Use a multiport manifold (a common piece of aquarium plumbing with one input and many outputs) to distribute the air to the different cultures. Some stiff plastic tubing (also available at the aquarium store) will allow you to inject the air at the bottom of each flask. But you should pump it through a filter with 0.5-micron openings, such as SLFH05010 from Millipore ($79 for a 10-pack; 800-645-5476), to keep bacteria from invading your sterilized containers [see illustration on opposite below].