Leptospira interrogans, the causative organism of leptospirosis, is characterized by a fine helical morphology, and the helix is almost always right-handed. However, one of the striking features of recent isolates of L. interrogans in Korea was the heterogeneity in their morphology. Even under optimal culture conditions($30^{circ}C$, EMJH medium), rods, spiral forms with right or left-handed helices, and even spherical forms of L. interrogans were present. Although the literature notes the presence of left-handed helices, long rods, and spherical forms in cultures of L. interrogans isolates, little is known about the cause of this morphologic heterogeneity. In an attempt to answer this question, this study was initiated to examine the effects of culture conditions, especially temperature and medium, on the morphology of L. interrogans. Four temperatures($5^{circ}C$, $15^{circ}C$, $30^{circ}C$, and $37^{circ}C$) and two types of media(Fletcher and EMJH) were used; one strain from Korean isolates and L. interrogans serovar canicola obtained from the Pasteur Institute(Paris, France) were employed throughout the study. The findings are as follows: 1. The L. interrogans isolated in Korea(UM-19) had a larger cell diameter($0.25{sim}0.30;{mu}m$: $0.10{sim}0.15;{mu}m$), and helix diameter($0.10{sim}0.60;{mu}m$: $0.10{sim}0.15;{mu}m$) than that obtained from the Pasteur Institute, but they varied in their distances between the helices($0.31{sim}1.00;{mu}m$: $0.50{sim}0.70;{mu}m$). 2, When UM-19 was grown at $37^{circ}C$ after months or longer preincubation at $5^{circ}C$ or $15^{circ}C$, the majority of the organisms were spiral forms; however, they became rods when subcultured at $30^{circ}C$ or $37^{circ}C$. No significant morphological differences were found between Fletcher and EMJH media. 3. When L. interrogans serovar canicola was subcultured more than ten times at $37^{circ}C$, some of the organism lost their motility as well as the hooks at either one or both ends, but only in Fletcher medium. The number of variants increased with the frequency of subculturing. These findings suggested that L. interrogans strain (UM-19) is different, in their morphology, from that of the Pasteur Institute, and its various morphologies may represent stages of the life cycle and vary with incubation temperature.