Most of the hybrids were recognized as unusual at the time of capture, and were saved as part of numerically selective samples from the streams (rather than being discovered in the laboratory, in random samples). Our measurements were made by methods defined by Hubbs and Lagler (1958); values are expressed as thousandths of the larger dimension. =_Chrosomus erythrogaster_ × _Notropis cornutus frontalis_=: KU 3872 (26.7 mm. in standard length) and KU 4170 (46.6 mm.) from Deep Creek, Riley Co., Kansas, Sec. 23, T. 11S, R. 7E, Dec. 14, 1957, and Apr. 26, 1958, respectively; and KU 4185 (39.3 mm.) from Bluff Creek, Pottawatomie Co., Kansas, Sec. 15, T. 6S, R. 8E, June 29, 1958. Compared in Table 1 with five specimens of _C. erythrogaster_, KU 3914 (39.3 to 47.3 mm., mean 43.0 mm.) from the same locality and of the same date as KU 3872 (above); and with five specimens of _N. c. frontalis_, KU 4184 (41.0 to 46.5 mm., mean 42.5 mm.) from the same locality and of the same date as KU 4185 (above). This cross has previously been recorded by Trautman (1957:326, 355) and by Minckley (1959:431). The head-lengths of the hybrids are greater than in specimens of like size of _C. erythrogaster_ or _N. c. frontalis_ (Table 1). Hubbs and Miller (1943:373-374) reported that hybrids of _Gila orcutti_ × _Siphateles mohavensis_ have larger, more robust heads than either of the parental species, perhaps because of heterosis. The enlarged heads in hybrids of _C. erythrogaster_ and _N. c. frontalis_ result primarily from elongation of the postorbital region, with lesser elongation of the snout and orbit. The enlarged head affects measurements obtained for other structures that are parts of the head (and expressed as proportions of standard length or head-length), causing a tendency toward _N. c. frontalis_ when the head-part is divided by standard length, and greater intermediacy or a tendency toward _C. erythrogaster_ when the head-part is divided by head-length. In characters in which the parental species differ most (size of eye, length of upper jaw, and width of gape), the hybrids are intermediate between the parental species, regardless of whether the measurements are expressed as proportions of head-length or standard length; however, tendencies toward one or the other of the parental species (dependent on the divisor) can also been seen in these characters. Some experimentally propagated hybrids show highly variable, and sometimes extreme characters, rather than intermediacy of meristic and proportional characters (Hubbs, 1956). TABLE 1. COMPARISONS OF THREE SPECIMENS OF CHROSOMUS ERYTHROGASTER × NOTROPIS CORNUTUS FRONTALIS WITH SPECIMENS OF THE PARENTAL SPECIES (MEANS ARE ABOVE, RANGES IN PARENTHESES BELOW)