Shoals of fish are often composed of individuals similar in body size or other characteristics. This study tested for the assortment by sex (controlled for body size) in a species with a low degree of sexual dimorphism. Zebrafish (Danio rerio) shoaling was studied in the laboratory using four different experiments utilizing two different methodologies. In most of the experiments, females showed a statistically significant preference for shoaling with other females, but males typically showed no preference. In choice experiments zebrafish preferred swimming near a compartment containing another zebrafish compared to swimming near an empty compartment, but this preference was strongest for same-sex pairings, both female-female and male-male. In additional choice experiments, the results showed a preference among females (but not males) for a group of three same-sex fish over three opposite-sex fish. There was no consistent result when females were forced to choose between three females and six males, thus the preference for same-sex shoalmates may be balanced with a preference for larger shoals. The second methodology used freely-swimming groups of four fish. Females swam closer to females than to males, but males showed no preference. Few previous studies have demonstrated sex-assortative shoaling.