Fat (per cent F) and protein (per cent P) contents, and the ratio between both (F-P) regarding bulk milk of commercial flocks of Latxa breed have been analysed. An increase of per cent F and per cent P throughout the milking season was observed in the different groups of flocks formed according to the characteristics of the lambing distribution. Nevertheless, milk produced by flocks with more scattered lambings tended to maintain a more constant composition. Those with more concentrated lambings, which were located in the highlands of the Basque Country, produced less concentrated milk (p less than 0.05), especially at the beginning of the milking season. The main management traits related to bulk milk composition, its evolution throughout the milking period, and differences between groups are discussed.