The size distribution and numbers of oocytes were examined in samples of ovaries from two sites and in fish in tanks whose past spawning activities were known. In an upland stream in northern EnglandCottus gobiowould seem to be an annual spawner, maturing a single batch of eggs each year. In a more productive stream in southern England, where the reproductive life is short, fish mature and lay a succession of batches of eggs during a long spawning season. The physiological and ecological implications of these characteristics are discussed.