ABSTRACT Interstitial animals were collected during spring, summer, and autumn from eight sampling sites along a Rocky Mountain river to examine small scale patterns of diversity, abundance, and faunal composition across the groundwater/surface water ecotone. At each site samples were taken from benthic habitats (superficial bed sediments), hyporheic habitats (underflow 30 cm below the bed surface), and phreatic habitats (30 cm below the water table of adjacent alluvial bars). Total taxa decreased markedly along the epigeanhypogean gradient. Crustaceans and insects (the majority of animals in all three habitats) progressively increased (Crustacea) or progressively declined (Insecta) in relative abundance along the epigean-hypogean gradient. Most of the 142 common taxa exhibited one of four types of distribution patterns along the gradient (hypogean, epigean, transitional, eurytopic). This study demonstrated a marked faunal gradient on a scale of meters across the groundwater/surface water ecotone. Faunal similarity coefficients (e.g., Jaccard) and detrended correspondence analysis of faunal distributions may provide comparative measures of ecotone permeability between physically contiguous interstitial habitats.