We surveyed pup survival in Mediterranean monk seals (Monachus monachus), at Cabo Blanco Peninsula (Western Sahara-Mauritania) colony from May 1993 to December 1997. This species breeds and hauls out on beaches inside two main caves. During the study period we detected a total of 93 pups that died or disappeared. The survival rate of 84 pups through the age of first moult (approximately two months) was 0.47. This value is similar to those reported for other pinnipeds breeding in caves but lower than for those breeding on open beaches. Mortality varied seasonally and appeared to increase as a result of storms, large ocean swells, and high tides. Mother-pup pair separation (and resulting pup starvation) and physical injury caused by impact against the rock walls of the cave and cliffs were established as the causes of most deaths. Beach surface area inside the caves also appeared to be a mediating factor in the effects of sea conditions. High pup mortality may be a limiting growth factor in this population, although cave dwelling protects the population from predators and human disturbance.