Demographic aspects of the encrusting bryozoan Celleporella hyalina (L. ) were investigated both at the colonial and the zooidal level. In the Menai Straits C. hyalina is the commonest epiphyte on the fronds of Laminaria saccharina (L. ) Lamour. The life-span of C. hyalina on this substratum ranges from a few days to 5-6 months according to season and settlement location, substratum abrasion being the main mortality factor. Experimental rearing in the natural environment showed that on a permanent substratum C. hyalina survives more than 18 months. From April to September C. hyalina invests, at simultaneously high rates, in somatic growth and sexual reproduction, whilst during the rest of the year investment is mainly in growth, but at a low rate. In comparison with colonies experimentally grown in restricted water flow conditions, colonies of C. hyalina in higher water flow regimes achieved: (a) bigger colony size, (b) higher reproductive output (RO) (reproductive zooids/autozooids) and (c) higher investment in female functions. RO increases asymptotically as body size increases and can be experimentally depressed by reducing water flow, which presumably diminishes the supply of food to the colony. Colonies of C. hyalina start sexual reproduction at a small size and do not delay sex in favour of achieving a bigger size, probably because: (a) unpredictable lifeexpectancy in the natural environment favours an early reproduction, (b) viable larvae need brooding for 3-4 weeks and (c) the modular construction allows partitioning of functions among zooids in the colony, minimizing interference between sexual activities and colonial growth. Resources are probably allocated to growth and reproduction in a similar way in other species of colonial organisms. Increasing the number of neighbouring colonies of C. hyalina reduces size and R0. Colonies isolated from foreign sperm produced larvae up to 40 days after isolation, suggesting that either self-fertilization or storage of foreign sperm takes place.