ABSTRACT: Blooms of the toxic alga Karenia brevis, commonly referred to as ‘Florida red tides,’ occur along Florida’s west coast on a near-annual basis, causing massive fish kills. However, few quantitative data on the ecological effects of red tides on fish communities exist. We surveyed fish communities in 5 habitats within Sarasota Bay and the adjacent Gulf of Mexico during the summers of 2004 to 2007 using a purse seine. We collected contemporaneous data on fish densities, fish species composition, K. brevis cell densities, water temperature, salinity, dissolved oxygen, and turbidity. Fish density (catch per unit effort [CPUE]) and species richness were significantly lower in all habitats during red tides. Shannon-Wiener diversity indices were significantly lower in 4 of 5 habitats during red tides. Classification and regression tree analysis showed significant negative relationships between K. brevis density and non-clupeid CPUE, and between K. brevis density and species richness. Fish community structure differed significantly between red tide and non-red tide conditions. Canonical correspondence analysis showed that of all the environmental factors investigated, K. brevis density had the greatest influence on community structure. Most trophic guilds were negatively associated with K. brevis density, whereas the guild that included clupeids was positively associated with K. brevis density. Florida’s fish kill database showed that 96% of local fish kills during 2003 to 2007 occurred during red tides. We concluded that red tides caused the observed changes in fish abundance and community structure. Harmful algal blooms occur throughout the world and may play an important, yet little understood, role in regulating fish communities.