Although sympatric populations of sticklebacks occur commonly along the north‐eastern coast of North America, very few studies have considered the interspecific relationships within such populations. This paper investigates prey selectivity, feeding behaviour and potential prey availability within sympatric populations from five brackish‐water sites in New Brunswick. Differences between the diets of species from different sites were related to differences in the range of prey available. Fish collected from sites with abundant aquatic vegetation consumed greater numbers of prey than those from vegetation‐poor sites. The proportions of prey types eaten by both adults and juveniles reflected prey species abundance at the different sites; thus shifts in diet over time were found to be related to changes in the abundance of potential prey and not to shifts in prey preference on the part of the fish. The size of prey consumed was related to mouth size; adult and juvenileGasterosteus wheatlandi, having the smallest mouths, consumed the smallest prey. Differences in prey size detected in the four species studied were due to differences in the types of prey consumed. Laboratory experiments on feeding behaviour showed that adultApeltes quadracusandPungitius pungitiuswere more efficient at capturing benthic prey than wereGasterosteus aculeatusandG. wheatlandi.Competition for food is thought not to occur in these populations, because of the abundance of potential prey and the morphological constraints on feeding beha