SUMMARY.1. Colonization of microhabitat implants by the amphipodGammarus pseudolimmnaeusin a small southern Ontario stream was studied in order to analyses the factors controlling habitat selection. The variables substrate particle size, current speed, presence of food and light were used in an analysis of covariance, with percentage weight of organic matter of silt and percentage interstitial space occluded by silt as the covariates.2. Greatest numbers of amphipods settled on microhabitats featuring large substrate particles, no current and presence of food. There was also a positive relationship between total numbers and the volume of silt deposited on the microhabitats by the stream; small quantities of silt had a beneficial effect on colonization but larger quantities became detrimental.3. The change from a positive effect occurred at approximately 25% occlusion of the interstitial space in large gravel (x̄ diameter=3.2 cm) and at approximately 55% occlusion in small gravel (x̄ diameter = 0.34 cm).4. Large animals (6–16.0 mm long) were found predominantly in microhabitats featuring food and large substrate. Medium‐sized animals (3–6.0 mm) were most commonly associated with no current and presence of food, and were positively affected by the amount of silt but, at the same time, were negatively affected by increasing occlusion of interstitial spaces by silt. Numbers of smallGammarus(<3.0 mm) were affected only by silt and in a similar manner to medium‐sized animals.5. Amphipod biomass was greatest in microhabitats featuring food and no current. Previous data on the behaviour of this species in laboratory stream‐tanks are compared with the microhabitat sele