Animals are known to derive benefits from associating with familiar individuals, and familiarity is important in the structuring of animal groups. In fish, individuals are known to shoal preferentially with others they have previously spent time with (familiar individuals). One mechanism used in fish shoaling decisions may be local olfactory cues; individuals prefer to shoal with conspecifics that have experienced a similar recent environment to themselves. We investigated the role of diet-based cues in the social decisions of domestic guppies, Poecilia reticulata. Diet strongly affected shoaling decisions. Fish fed a bloodworm diet preferred to shoal with familiar individuals and with those that had been fed a similar diet. Flake foodfed fish, on the other hand, preferentially associated with fish fed on bloodworm, and showed a strong preference for unfamiliar fish when both shoals were fed flake food. These results suggest that several factors may interact to influence shoaling decisions. Bloodworm-fed fish also strongly preferred the olfactory cues of their own diet to flake food odour whereas flake food-fed fish showed no preference for either odour type. Fish on bloodworm diets, however, grew faster. We suggest potential explanations for the flake food-fed individuals' preference for unfamiliar fish.