To understand animal behaviour, it is important to consider the environment in which it occurs. The environment, consisting of both abiotic factors and social context, is usually highly variable and leads to variation in individual’s and group’s behaviour. To better understand the environmental influences on behaviour of pygmy halfbeaks (Dermogenys collettei), a small live-bearing fish, I viewed videos of shoals of wild halfbeaks in Singapore. I investigated effects of environmental variation (water depth, canopy cover and water vegetation) and social environment (group size and male to female sex ratio) on halfbeaks’ aggression and mating behaviours. I found that environment had little effect and most variation between studied shoals was probably due to social factors. I found some evidence for aggression increase in larger shoals, primarily in males. Sex ratio had different relation with aggression for individual sexes and mating behaviours decreased when sex ratio became more male biased. This study shows that halfbeaks probably modify their behaviour in relation to social environment. I also show that these changes are similar to those described in other species, which strengthens the validity of using halfbeaks to study social interactions.