Coral mucus-degrading bacteria were isolated by an enrichment culture procedure. The isolates were able to grow as pure cultures on 10% sterilized mucus in seawater, yielding 10(8) CFU/ml. The isolates, mostly Vibrio strains, were classified by classical and molecular methods. When carbon, nitrogen, and phosphorus compounds were added separately to the mucus medium, there was no increase in CFUs; however, when they were added together, there was a large increase in cell yield. The indigenous bacterial population of coral mucus increased from 10(3) to 10(8) CFU/ml when incubated at 30 degrees C for 11 h, changing from a heterogeneous community to a Vibrio-dominated population. Factors which regulate the abundance and diversity of coral mucus bacteria are discussed.