Larval fish were collected in the Lower Mississippi River at Sunnyside Revetment on 27-28 June 1978 to investigate diel fluctuations in diversity and ahundance and to determine whethersamples collected for 1-2 hours and from opposite sides of the hoat can he considered replicate samples for a time period. Numher of taxa and density differed significantly (P < 0.01) hetween time periods of the diel cycle. Numher of taxa and density were greatest at dusk. Twelve taxa were collected only in dusk, night, and dawn samples. Aplodinotus grunniens, Carpiodes spp. and c1upeids accounted for 95 percent of the larvae collected. A. grunniens and Carpiodes spp. were more ahundant in dusk and night samples. Clupeids were more ahundant diurnally. Variance component analyses indicated that samples collected for 1-2 hours during each time period can he considered replicate samples for diversity and ahundance estimates for that time period and samples collected from opposite sides of the hoat can he considered replicate samples for diversity hut not density estimates for each time period.