Abstract
Multiple regression analysis was used to determine the environmental factors which affected the microdistribution of different sizes of G. pulex (L.). Median particle size was linearly related to mean Gammarus size and was the most important of the factors, which together accounted for 54.3% of the variation in mean Gammarus size. Males in precopula were found in particle sizes smaller than expected and unlike females in precopula, non-breeding Gammarus and juveniles, their size was not correlated with particle size. Breeding females, non-breeding animals and juveniles select substrates of a particular size; sexually active males select areas containing receptive females.