To date there has been only on major study concerned with parasites of freshwater fishes of California. E. C. Haderlie (1953) summarized investigations up to that year and conducted a general survey of the monogenetic and digenetic trematodes, cestodes, nematodes, acanthocephalans, copepods, and hirudinians of fishes of Northern California. From 2010 fishes representing 36 species of 11 families examined over a three-year period, he obtained a total of 59 species of helminth parasites, copepods, and hirudinians, which include 20 species of digenea and 16 species of cestodes. In addition to the taxonomic study, Haderlie attempted to correlate the relative occurrence of the parasites with various ecological habitats. These data are incomplete, except for a general ecological discussion of the parasites taken from Clear Lake and its contributing streams. In the Sacramento-San Joaquin area the monogenetic trematodes are the only group that has been extensively studied. This work has been done by Dr. J. D. Mizelle of Sacramento State College. The primary purpose of the current investigation is to gain some knowledge of the species of endoparasites of fishes of the San Joaquin Delta. Two hundred and thirty sic fish were examined, including diadromous, potamodramous, anandromous, and territorial species of San Joaquin Delta. This has resulted in the recovery of two previously described and one new adult digenea, three metacercariae, two adult and three larval cestodes,and two cestodarians. Not included in this study are the Acanthocephala and Nematoda. A few cestodes are also not included because of their poor condition. The Host-Parasite List (p. 49) of this paper gives a summary of the fishes examined by Haderlie (H) and the present author (E) with the number of each species examined and the species of trematodes and cestodes recovered.