We investigated the productivity of nymphs of the mayfly Hexagenia limbata in Lake Waco, a central Texas reservoir, and assessed its association with chlorophyll content of the water . We hypothesized that food availability measured as chlorophyll content of the water may directly associate with growth of Hexagenia and predict population productivity. To test this, we compared production by mayfly populations at two stations in the same reservoir ; a northern station receiving water input with high chlorophyll content, and a southern station receiving water with low chlorophyll content . Both stations had similar substrate type and abundant mayflies . Benthic samples were collected from October 1984 through September 1985, and dissolved oxygen and temperature of the water were monitored .Annual production (size-frequency method) was 1270 mg m -2 (P/B = 7 .5) at the northern station and 1990 mg M-2 (P/B = 6 .1) at the southern station . The mean standing crop was 323 mg m -2 at the southern station and 169mg m -2 at the northern station . Densities of mayflies at the two stations were not significantly different .Mean chlorophyll concentration (total mg pigment) during the sampling period was 23 .5 mg m -3 at the northern station and 16 .7 mg m -3 at the southern station . Therefore, the station with lower mean chlorophyll content had higher secondary productivity by Hexagenia . Conversely, the station with higher mean chlorophyll content had lower mayfly productivity . The productivity of the mayfly populations did not positively associate with the chlorophyll content of the water, and chlorophyll content did not predict the success of the population of Hexagenia . Variation in mayfly growth success was associated with differences in temperature and dissolved oxygen . The northern station with higher chlorophyll content and lower productivity had low dissolved oxygen and temperatures higher than optimum for growth .