The maternal social spider Coelotes terrestris demonstrates extended care towards its progeny: the mother guards its egg sac for 3–4 weeks, then stays with its young from the time of their emergence until their dispersal about 1 month later. The present investigation evaluates the adaptiveness of these maternal behaviours by comparing the fitness of females performing them with that of females separated from the egg sac or the spiderlings. By protecting their egg sacs from predation and parasites, and by pursuing this task while supplying the young with food, mothers enhance the survival rate and the development of many of their spiderlings. The costs linked with these activities, estimated by the ability to produce another clutch, appear variable according to the stage in the reproductive cycle. In such terms, the egg sac guarding appears to have a low cost in relation to the care given to the spiderlings.