Agglutination serotypes were investigated by testing mycobacterial cultures from various sources with a large battery of antisera raised in rabbits. Absorption of agglutinins was performed when necessary to clarify serological relationships. Types in addition to those already described were recognized as follows: Mycobacterium scrofulaceum, serotype Cole; Mycobacterium gordonae, seven serotypes (34 strains isolated from various water sources were serotype Marshall); Mycobacterium marinum types 1 and 2; and Mycobacterium xenopi. It is proposed that these types be numbered, respectively, as follows: Mycobacterium avium complex, serotype 44; M. gordonae, serotypes 1 to 7 (Kowal, Marshall, Szent, Puntal, Moore, Lanton, and Brown, respectively); M. marinum, serotypes 1 and 2; and M. xenopi. The strains of M. xenopi included in this study could not be separated into different types by the techniques utilized.