Mixed cultures derived from Boston Harbor sediments, which gave evidence of methylating tin compounds, yielded two isolates, each aBacillussp., which methylated CH3SnCl3to form (CH3)3SnX in coculture. Neither organism alone gave evidence of methylation. SnCl4and (CH3)2SnCl2were not methylated. Transfer of either organism to filtered medium containing CH3SnCl3in which the other had grown did not lead to methylation. Therefore methylation by the coculture is not simply a matter of cross‐feeding.