The seasonal abundance and distribution of the native caridean shrimp, Crangon franciscorum, and the introduced shrimp, Palaemon macrodactylus, in the channel areas of the San Francisco Bay Delta were studied from April, 1977, through October, 1978. C. franciscorum reproduces earlier in the year and grows to a larger size than P. macrodactylus. C. franciscorum reproduction occurs from December to June in the high salinity waters of San Francisco Bay and the Pacific Ocean. P. macrodactylus reproduction occurs from May to September in the delta as well as in higher salinity habitats. Length-weight and length-fecundity relationships differ significantly between the two shrimp. Both shrimp are limited upstream by low salinities, few shrimp occurring at salinities \< 1\textperthousand. The downstream distribution of these shrimp is related to prey availability, i.e., Neomysis mercedis abundance.

Indices of spatial overlap, or interspecies patchiness, indicate a high degree of overlap which varied seasonally and exhibited markedly different patterns in 1977 and 1978. Directional crowding (intraspecific patchiness) also differed between 1977 and 1978. P. macrodactylus appears more tolerant of varied environmental conditions than C. franciscorum, occurring in the same habitats and also in additional ones not utilized by C. franciscorum. This may give P. macrodactylus a competitive advantage when trophic resources become limiting.