A total of 25 marine caulobacters were isolated from littoral marine sources. Several aspects of their physiology and morphology were examined, as well as their suitability for genetic manipulation in laboratory cultivation. Caulobacters were readily isolated from all sources, including samples from areas containing pollution-related organic compounds. All isolates grew best in media containing seawater, but eight strains grew if sea salts were replaced with NaCl alone, three strains grew at 1/10 the normal sea salt concentration, and one isolate grew, albeit poorly, in freshwater medium. Of the marine isolates, 12 strains grew under anaerobic conditions, indicating that some caulobacters are not obligately aerobic bacteria, as they are currently categorized. Although some freshwater caulobacters are able to oxidize manganese, this capability was not found in these marine caulobacters. Of the marine isolates, 10 strains were resistant to mercury chloride concentrations 10- to 20-fold greater than that tolerated by sensitive bacteria. However, a mercury reductase gene comparable with that found in R100-type plasmids was not detected by gene hybridization. With respect to the potential for genetic experimentation, most strains grew rapidly (3- to 4-h generation time at 30 degrees C), producing colonies on solid media in 2 to 3 days. The isolates were sensitive to antibiotics commonly used in recombinant DNA experiments, and spontaneous drug-resistant mutants were selectable. Conjugal transfer of plasmids from Escherichia coli to several marine caulobacters was demonstrated for four broad-host-range plasmid incompatibility groups, by using both self-transmissible plasmids and cloning-oriented plasmids that require a helper plasmid. Conjugal transfer of broad-host-range plasmids between freshwater and marine caulobacters was also demonstrated in both directions. Native plasmids of approximately 100- to 150-kilobase sizes were found in 2 of the 25 marine Caulobacter strains. The native plasmids were present in relatively high copy number and appeared stable in laboratory culture. In short, the marine caulobacters appeared appropriate as candidates for genetic manipulation and the expression of selected genes in the marine environment.