Three varieties of resting stage, an exospore and two types of cyst, were formed by methane-utilizing bacteria. Exospores were budded off by two types of organism, both of which underwent a change in morphology prior to spore formation. Exospores possessed some properties in common with endospores : staining properties, some structural features, mode of germination, desiccation and heat resistance, but contained no detectable dipicolinic acid. Some organisms formed desiccation-resistant cysts similar in morphology, staining properties and fine structure to cysts formed by Azotobacter species. Other strains formed non-desiccation-resistant bodies considered to be similar to immature azotobacter-type cysts. A desiccation-resistant cyst formed by one strain differed in structure and appearance from Azotobactertype cysts and was referred to as a 'lipid' cyst.

S U M M A R YMethane-utilizing bacteria were examined by electron microscopy and found to possess complex membranous structures within the cytoplasm. Two types of membrane organization were recognized. One type consisted of pairs of membranes which either extended throughout the organism or were arranged at the periphery where they ran parallel to the cytoplasmic membrane. The other type consisted of vesicular discs of membrane organized into bundles which were distributed throughout the organism. Bacteria utilizing C, to C4 gaseous n-alkanes and C,, to C,, liquid n-alkane mixture did not possess such extensive membranous structures. The former contained membranous bodies of the mesosome type, whilst the latter possessed only a cytoplasmic membrane. These structural differences add to the growing list of properties separating CH,-utilizing bacteria from those utilizing C, and higher n-alkanes.

Slide cultures showed that growth of a hydrocarbon-utilizing Torulopsis sp. occurred only when the cells were in physical contact with the n-alkane droplets. Growth in batch culture was found to be linear and dependent on the surface area of substrate available to the cells. The hydrocarbon may enter the cell without first being converted to a more water-soluble intermediate.