A basic assumtion in the study of mating systems is that the amount and disribution of food and sexual partners influence differences in reproductive success. In this study the ecological, demographic and phenotypical determinants of variation in the social and genetic mating patterns and in reproductive success were analized for a population of Water Pipits Anthus spinoletta. The breeding habitat of the species lies in a variable alpine environment. Conditions at the study site varied markedly in time and space. This is due to a patchy vegetation pattern with large differences in food resources, an increasing population size over the study years and differences in nest predation. The operational sex ratio and the age distribution were stable between breeding seasons. Out of 278 social mating patterns studied, most were monogamous (86%), followed by bachelors (9%), polygyny (3%) and polyandry (2%). For paired individuals the fitness differences of the various mating patterns were small. The population showed an age-assorted mating system. Variation in reproductive success was best explained by nest predation and age of males and females. DNA fingerprints of 393 young from 95 nests revealed extrapair parentage in 7.1% of the young from 18.9% of the nests. Since there was no significant relation between territory-specific prey biomass and reproductive success, and the occurrence of nest predation was stochastic and unpredictable, age may be the most reliable criterium for mate choice. Under these circumstances, monogamy can be expected to be the dominant pair bond, especially under conditions where both parents are required to successfully rear the young.