Prey organisms of fish larvae vary in density, both temporally and spatially. Do fish larvae respond behaviorally to variation in food density? The responses of larval herring C. harengus L. to different densities of copepod nauplii (5 to 360 nauplii 1-1) were studied in laboratory experiments. Larval feeding rate increased as food availability increased until an asymptote was reached. At low food densities, larval swimming activity was .apprx. 100% higher than at the highest food concentrations; a change in larval swimming mode within the range of food densities was also noted. The water volume searched by the larvae, calculated on the basis of rate of feeding attacks, showed a pronounced increase when food density declined, partly due to the increased swimming activity but also as a result of an increase in distance of reaction to food particles. The adaptive significance of these behavioral changes is discussed.