Kleptoparasitic spiders are known to have a close association with host spiders, yet there have been few studies demonstrating how host availability influences the dynamics of kleptoparasites. Field surveys were conducted at five sites differing in host composition in sub-tropical areas in Japan, at about two-months intervals. Argyrodes flavescens and A. bonadea were both found more frequently on webs of two Nephila species than expected from the web areas they occupied among webs of all web spiders. Seasonal dynamics of Argyrodes changed greatly according to whether N. clavata was present or not, indicating the importance of Nephila on Argyrodes populations. The peak density of A. bonadea came earlier than that of A. flavescens. Because A. flavescens is known to limit the number of A. bonadea on host webs, the decrease in the density of A. bonadea may be due to the effect of interspecific competition by A. flavescens.