Fouling communities of commercial oyster crops in San Quintin Bay (Baja California, Mexico) were investigated to understand patterns of shell cover, species composition, and colonization. Historically dominated by soft sediment systems, San Quintin Bay currently supports a large oyster (Crassostrea gigas) aquaculture industry. Oyster shells are the main source of hard substratum in the bay, without which fouling communities would be mostly absent. Fouling communities are a nuisance to oyster farming because they result in increased handling time in cleaning and packaging oysters. To investigate the previously undocumented fouling communities of San Quintin, 3 sites within the bay were surveyed for 18 mo (2004 to 2005). Every month between July 2004 and December 2005 samples were deployed and collected so as to obtain shells that had been submerged for either 1, 2, 3, 6, or 12 mo. The majority of the fouling organisms were ascidians, with bryozoans, sponges, hydrozoans, and algae also present. Results show that fouling cover increases with length of time shells were submerged, with an average of ∼75% of shell surfaces covered by fouling organisms after 12 mo, the maximum time oysters are grown before harvest. Colonization in the system is variable throughout the year, with ascidian colonization being positively correlated with warmer water temperatures. Several of the species, mostly ascidians, of the fouling community are nonindigenous to the Pacific coast of North America and account for up to 40% of the total fouling cover. Fouling community cover and colonization is similar to what has been documented in other bivalve aquaculture systems. But, because most aquaculture ventures in San Quintin are small, family-based artisanal enterprises, the socio-economic impact of fouling is larger and more difficult to manage. Understanding patterns of fouling cover, colonization, and species outbreaks is necessary to develop feasible rearing solutions that can reduce the impact of fouling.