It is known from previous laboratory studies that female choice in the river bullhead, Cottus gobio, is mainly influenced by the size of the male, and that the presence of eggs in the nest also plays a role. However, the process by which females visit and choose among prospective mates is still poorly understood. We examined eight possible tactics of female choice through computer simulation, and we compared their fit to the reproductive patterns observed in two natural populations. Neither of the tactics proposed for other species, nor combinations of them, were able to predict accurately the field data, although one of them (fixed threshold) provided a fairly good fitting. Conversely, a new model (cluster sampling), which takes into account the spatial distribution of males, produced for both populations a set of predictions on the male mating success not significantly different from the data recorded in the field. Under this model, females sample more males when males are clumped, thus reducing substantially, on the average, the ratio between the distance travelled before spawning and the number of sampled males. When models incorporated the preference for already mated males, good fitting was obtained if females preferred nests containing eggs not older than 36 h, a finding that agrees with field results.