Marine fouling organisms occurring on test panels of various substances and at several locations and depths in the Monterey Habor, California, were studied for identification and significance. Some panels were immersed for the entire length of the study--June 10 to September 16, 1966; others, mainly plywood, were immersed only for month-long periods throughout the study. Barnacles, bryozoans, and serpulids were the major fouling organisms in the inner harbor, while hydroids were most significant in the outer harbor. The barnacles reached maximum attachment in June and July, but were covered later by bryozoans. Phoronid worms were abundant in August and September on the shallow panels in the inner harbor. Fouling increased with depth and distance away from direct sunlight. Fibrous masonite and wood panels were the best collecting surfaces and stainless steel the worst.