The level of natural hybridization in Lake Texoma between Dorosoma cepedianum and D. petenense for the years 1962-63 and 1968-69 was determined to be 2.5 and 1.7% respectively. The two species have been sympatric in that reservoir since 1957. Male and female hybrids with well developed gonads were collected. Eggs from a hybrid were fertilized experimentally with milt of D. petenense and produced viable larvae. Hybrid fry were produced in the laboratory by crosses between D. cepedianum males and D. petenense females but not the reciprocal. Spawning of the two species is discussed in relation to the possible means of natural hybridization. Natural hybridization between the clupeid species Dorosoma cepedianum and D. petenense was reported by Minckley and Krumholz (1960). They cited as the probable cause, the less abundant D. petenense joining groups of spawning D. cepedianum. Hybrids were first identified in Lake Texoma in 1960 by C. D. Riggs and G. A. Moore (personal communication). Dorosoma petenense was first collected from the lake in 1957, when the young-of-the-year were already more abundant than those of D. cepedianum (Riggs and Moore, 1958). The population of D. petenense expanded rapidly and soon surpassed that of D. cepedianum. At present, both species are abundant. This study was conducted to determine factors responsible for hybridization and if the relative abundance of hybrids changed over several years. Specimens were collected with experimental gill nets (19-, 25-, 31-, 38-, and 51-millimeters bar-mesh) and hybrids were identified from fresh material. Characteristics described by Minckley and Krumholz (1960) were easily recognizable. In general, hybrids had a snout shape similar to that of D. cepedianum but with the yellow pigmentation characteristic of the fins of D. petenense. It is assumed that these were probably fl hybrids and is not known if subsequent filial groups could have been recognized. Gill nets were selective for the larger specimens and D. petenense were regularly collected only in the 19 mm mesh. This is reflected in the small number of D. petenense collected (Table I) which would tend to increase the apparent level of hybridization. But since the method of collection was uniform throughout the study, the error would tend to be relatively constant. The level of hybridization during the initial year of introduction (1957) was not established but was estimated at TOughly 5-year intervals thereafter. The percentage of hybrids collected appears to have changed only slightly during the study period (Table I). Five years following the initial appearance of D. petenense, the level of hybridization was estimated to be 2.5%, after IO years it had decreased slightly to 1.7%,
