A new method was developed for determination of the doubling times of spiroplasmas. In this procedure, the time required for medium acidification of tubes in tenfold dilution series was recorded. Sixty-four spiroplasma strains, representing 24 groups and 11 subgroups, were studied. Eight strains representing putative new groups were also included in the study. Doubling times at 5, 10, 15, 20, 25, 30, 32, 37, 41, and 43°C were determined. The range of temperatures for spiroplasma growth was 5°-41°C. Twenty-three spiroplasmas had optima of 30°C, 29 had optima of 32°C, and 13 had optima of 37°C. The fastest growing spiroplasma was the MQ-4 strain (group XI), with a doubling time at optimal temperature of 0.6 h. The slowest was the Jamaican corn stunt strain B655 (subgroup I-3), with an optimal doubling time of 36.7 h. Spiroplasma strain B31 (group IV) had the widest range (5°-41°C), while the DW-1 strain and some subgroup I-3 strains had the narrowest, growing only at 25° and 30°C. Some spiroplasmas grew well at 41°C, but none grew at 43°C. The ability of spiroplasmas to withstand a wide range of temperatures may reflect the conditions to which they are exposed in nature, including the temperatures of the insect, tick, and/or plant hosts in which they are carried and the plant surfaces from which they may be acquired by arthropods.