A comparative study was undertaken of Amazonian fish diversity and density (abundance and biomass) in nutrient poor (blackwater) and richer (whitewater – várzea) habitats in the vicinity of Tefé, Brazil. The whitewater sampling sites, in the floodplain of the Rios Solimōes and Japurá, had high turbidity (Secchi disc 0.03–0.7 m), a conductivity of 64–110 μS cm–1 at 25°C and a pH of 6.6–6.9. The blackwater sites Lagos Amanã, Iamã and Tefé had a low turbidity (Secchi disc 2.0–2.2 m), a conductivity of 7–11 μS cm–1 at 25°C and a pH of 5.3–6.0. The fish communities of open water, floating meadow and forest margin were sampled. Both whitewater and blackwater sites held high diversity fish communities with many species in common. Whitewater habitats were more diverse yielding 108 species, compared with only 68 from blackwater. Fish density within floating meadow was estimated during high water April and May 1994, and low water October 1994 and March 1995. During the high water season fish biomasses in blackwater lakes (31.1 gm–2) were significantly greater than those of either whitewater lakes (13.4 gm–2) or whitewater river channels (3.45 g m–2). At low water, in October, whitewater channels were generally found to hold the highest fish biomass (204 g m–2) although in March a blackwater site yielded the highest fish biomass recorded (285.9 g m–2). The amount of floating meadow habitat within Whitewaters is however much greater than that in blackwaters. It is suggested that blackwaters may offer a stable habitat resulting in a high standing crop. In comparison, variation in conditions such as dissolved oxygen concentration in the whitewater may limit standing crop, but still allow periods of high productivity. Evidence was found for migration of fish from várzea lakes during periods of low oxygen availability. The high diversity and biomass of fish caught in both whitewater and blackwater lakes indicates that water acidity and conductivity are poor predictors of fish diversity and density in tropical floodplain lakes.