This study investigated the factors affecting different group sizes of Maasai giraffe (Giraffa camelopardalis tippelskirchi) in the Serengeti-Ngorongoro ecosystem. The study was motivated to test the following hypotheses: 1) in a given group, the sex ratio of males to females with calves is related to group size; 2) group size affects vigilance behaviour; 3) illegal hunting influences group size and vigilance behaviour; and 4) group size differs in different habitats, being larger in woodlands during the wet season and in riverine habitats during the dry season. A negative relationship between the ratio of the proportion of adult males to females with calves and group size was observed, whereas the proportion of females and calves increased with group size. The proportion of vigilant individuals decreased with an increase in group size. Habitat, risk of illegal hunting and proportion of calves in the group was found to be significant contributors to an increase in vigilance behaviour. However, the results do not support the hypotheses that seasonality and habitat preference affect group sizes of the Maasai giraffes. Emphasis should be put on antipoaching efforts on males and nursery groups, especially in areas with a high risk of illegal activities.