C, bacteria oxldlzing methane and monomethylamine (MMA) are readily enriched from seawater and associated manne materials, but only Methylomonas pelagica oxidizing methane and Methylophaga marina oxlhzing MMA have been adequately characterized. Antisera with good specificity were prepared against these taxonomically and trophlcally different oceanic methylotrophs as well as against methanol dehydrogenase, a key enzyme presumably present in all aerobic methylotrophs. These antisera were used in an indrect imrnunofluorescence procedure to compare the relationship of these type oceanic species to wild-type cultures from the estuarine waters of Chesapeake Bay (USA) and the open sea. Of the 32 methanotrophic enrichments attempted from the Sargasso Sea, 23 (72 %) yielded methane-oxidizers. Of the 27 isolates obtained in pure culture, 25 (96%) were identical to Methlyomonas pelagica. By contrast, none of the 54 estuarine methaneoxidizers from Chesapeake Bay were identical to M. pelagica. 13 % were related and 87 % were unrelated. The anti-MDH (methanol dehydrogenase) serum reacted to 23 O/a of the wild methanotroph cultures. Of the 18 oceanic methylaminotroph enrichrnents, 44% were indistinguishable from Methylophaga marina, 6 % were related, and 50 % were unrelated. In contrast, of the 41 Chesapeake Bay MMA-oxidizers, 12 % were indistinguishable from M . manna, 17 % were related, while 71 % were unrelated. The anti-MDH antiserum reacted with only 15 % of the wild MMA-oxidizing bacteria. The implications of the taxonomic affinities and trophic requirements of the methylotrophs to their estuarine and oceanic distribution and to their anaerobic methanogenic bacterial consorts are discussed.