AbstractA quantitative investigation of the spawning of the sand goby, Pomatoschistus minutus and the common goby, P. microps (Gobiidae, Pisces) on a shallow soft bottom was carried out between 3 June and 8 August, 1980. Sixty randomly sampled 1 m² quadrats were visited every week and the number of nests counted. P. minutus had its spawning maximum two weeks before P. microps, according to calculation of the arithmetic mean. Generally P. microps spawned in shallow water of less than 75 cm depth and P. minutus spawned below 75 cm. A clear difference was noted between the species in the substrate used for nesting. P. microps preferred shells of Mya arenaria and P. minutus used pieces of wood. In an experiment in which artificial substrates were provided the number of occupied nests m^-2 was 11 to 18 times greater than the natural values. This indicates that a lack of suitable substrates limits nesting in the area. P. minutus took greater advantage of the artificial nests (the number of occupied nests was 20 times greater than natural abundances) than P. microps (in which the number of occupied nests increased about 10 times). In spite of the apparent lack of suitable substrates for nesting, competition was minimized as the two species used different substrates, water depths and times for nesting.