Several methanotrophic microorganisms, i.e., Methylococcus capsulatus (Bath), Methylomonas albus (BG-8), Methylosinus trichosporium OB3b, and Methylocystis parvus (OBBP), were evaluated for growth and methane utilization. The effect of temperature was examined in the range of 25 to 45 degrees C for growth and methane utilization. The temperature variations (25-35 degrees C) had minimal effect on growth of M. albus and M. parvus. Methane consumption varied at different temperatures with a maximum of 0.67 mol%/h and 0.53 mol%/h. at 30 and 35 degrees C, respectively, for M. albus and M, parvus. The growth and methane consumption was slower for M, trichosporium OB3b as a maximum methane consumption of 0.07 mol%/h was obtained at 25 degrees C and growth was inhibited at 35 degrees C. M. capsulatus grew the best at 37 degrees C and growth was affected at higher temperature of 45 degrees C. Of the different cultures examined, M, albus and M, capsulatus grew the best and were further evaluated for the effect of pressure in the range of 10-50 psi. The results obtained using M. albus demonstrated an enhancement in methane consumption rate by fourfold and final cell concentration by 40% at a pressure of 20 psi by injecting a methane/oxygen mixture, however further increase in the pressure up to 50 psi inhibited the growth. The inhibition was not seen with nitrogen incorporated mixture of oxygen and methane, which suggest that the high partial pressure of methane and/or oxygen are inhibitory for the growth of M. albus. M. capsulatus was more sensitive to pressure as evidenced by inhibition at the relatively low pressure of 10 psi.