This study tested the association between variation in precipitation and behavioural seasonality in Sotalia guianensis, at the Curral inlet (6º13′00″S 35º3′00″W) in north-eastern Brazilian coast. From October 1999 to September 2003, observations were carried out four days a month. Behaviours were registered instantaneously each five minute interval. The observation period consisted of six hours into the daytime (0600 to 1700 h). Two observers inspected the area simultaneously, from a fixed location at the top of the cliff. During June to November, socialization (MdJN = 1.524; MdDM = 0.667; U = –2.848; P = 0.004) and forage (MdJN = 3.268; MdDM = 1.762; U = –2.881; P = 0.004) were significantly higher than from December to May, although the number of individuals (MdJN = 6.000; MdDM = 5.000; U = –1.395; P = 0.163) did not change over the year. Such behavioural seasonality was not associated with dry/rainy periods (number of individuals: MdRS = 5.000; MdDS = 6.250; U = –1.672; P = 0.095; socialization: MdDS = 0.947; MdRS = 1.464; U = –0.858; P = 0.391; and forage: MdDS = 3.017; MdRS = 2.292; U = –1.506; P = 0.132). These results support behavioural seasonality in an equatorial odontocete population.