During the winter months, edible crabs Cancer pagurus and velvet swimming crabs Necora puber from the English Channel can harbour infections by a Hematodinium sp. dinoflagellate. This parasite is responsible for a highly pathological condition known as 'Pink Crab Disease' (PCD) in the edible crab. In the current study, a high proportion (between 25 and 100%) of Hematodinium-infected edible and velvet swimming crabs captured from 2 sites in the English Channel also harboured a systemic infection by a yeast-like organism. This is the first report of such an infection in crabs. Budding yeast-like cells were observed intracellularly in circulating haemocytes and free in the host plasma. These cells stained positively with silver and periodic acid-Schiff (PAS) reactions. Despite an apparent haemocytopoenia in Hematodinium-infected crabs, haemocytic encapsulation of yeast-like cells was evident, while no such response was observed against Hematodinium sp. plasmodia. It is hypothesised that Hematodinium infection in these crabs may either increase the likelihood of secondary infections via an indirect suppression of the host immune system, or alternatively, decrease the likelihood of competitive growth inhibition by stimulating the host immune system to encapsulate and destroy secondary pathogens. Results are discussed with regard to the likely identity of the yeast-like organism, and the role of secondary pathogens in the eventual mortality of Hematodinium-infected hosts.