Nitzschia alba (Bacillariophyceae), a colourless (apochlorotic) pennate diatom, was found on the surface of seaweeds collected from the Clyde Sea area between August and December, 1994. This is the first record of N. alba in UK waters. Nitzschia was found to replicate much more rapidly than heterotrophic protists of similar size. At 20°C, cells divided in just 6.9 h and even at 5°C the generation time was only 38.2 h. The ecological importance of these diatoms has been overlooked by microbial ecologists despite the high productivity of coastal ecosystems and recent work that has shown that Nitzschia can utilize seaweed phycocolloids and invade damaged seaweed tissue.