The breeding biology of a southern population of the little-known Savi’s Warbler Locustella luscinioides is described and compared with other parts of its distribution and other species. Males arrived at the Salreu marshlands, Portugal, in the second week of March whereas females arrived 6–10 days later. Early-arriving males bred earlier and had greater mating and breeding success. Female breeding success also declined with first-egg date. The return rates of males, females and juveniles were 27, 14 and 2%, respectively. Clutch and egg sizes were smaller than in other populations studied; the first decreased and the latter increased within a season. Egg size was correlated with female tarsus length, but not with other body size measurements. The differ- ences in egg size between populations seem to be fully explained by dif- ferences in female body size. A Mayfield estimate of the probability of nest survival was 40% overall, and was higher during the incubation period (74%) than during the nestling period (57%). Nest predation was the major cause of nest loss. Differences in clutch size and preda- tion rate explain the differences in breeding success between popula- tions. Females produced on average 5.4 fledglings per season, which is enough to balance the mortality rates often found in other passerines.