In altricial mammals, mothers' care and attendance are essential for young to acquire survival skills. Despite the crucial importance of this early period for pup survival, not much is known about motherpup behaviour and time allocation in the wild. To improve our understanding of the critical period of the sea otter development, a total of 240 30-minute focal follows of females with pups of different ages were conducted in four 6-hour-periods (dawn, day, dusk and night) from 4 June to 12 August 2008. Generalized linear models were used to investigate the impact of age, weather, tide and time of day on otters' behaviour. Pups significantly decreased resting behaviour and increased feeding, travelling, interacting and grooming behaviour with increasing age. Females with large pups fed and interacted significantly more, and travelled and groomed the pup less than females with small pups. Foggy conditions were associated with less resting on the water and increased travelling by pups. Tide did not appear to have an effect on sea otter behaviour. Grooming by pups occurred mainly during dawn in medium sized pups whereas large pups groomed themselves mostly during the day and night. Variations due to times of day tended to be apparent only in larger, i.e. older pups. Females and large pups were more active during the day. Mothers of smaller pups spent most of their active time travelling whereas females with large pups were mostly feeding. This study showed how sea otter females with pups adapted their behaviours as their pups grew and matured during the first months of life.