This study compared the fecundity of the colonies of the cheilostome bryozoan Cribrilina annulata (Fabricius, 1870) collected from two differing substrates - shingles and the algae Odonthalia dentata (Linnaeus) Lyngbye, 1819 - in the White and the Barents Seas. Almost all colonies growing on algae had brood chambers (ovicells), and their numbers per colony were high. In contrast, colonies with ovicells were rare on shingles, having fewer brood chambers. Colonies growing on O. dentata started reproduction at a smaller size (having fewer zooids per colony) than those on the shingles. These data contradict the generally accepted opinion that stable substrates are favourable for bryozoan sexual reproduction. We also found that ancestrulae are larger in the colonies growing on shingles, which suggests that the larval pool might be subdivided into two 'subpopulations'. Conditions for such subdivision are discussed.