_H. g. gulonella_ occupies small rivers and creeks, preferring pools with moderate currents. In fall, dense concentrations of this subspecies have been found in small pools, where brush, driftwood or other debris deflects the current and prevents filling with drifting sand. Hundreds of flathead chubs were collected in such pools in the Purgatoire and Arkansas rivers. Specimens were also collected with ease in Beaver Creek, Colorado, from pools with murky water and slight flow, over bottoms of gravel and bedrock. No brush or other debris was near the pools. In each case the streams carried little water, although they undoubtedly carry greater volumes of water in spring and early summer after rains and spring thaws. The preferred bottom-type for this subspecies seems to be gently shifting sand. _H. g. gulonella_ is found in warm-water streams, whereas _H. g. gracilis_ occurs in cooler water. The southwestern subspecies was taken in August in the Mora River drainage at Sapello (temperatures above 80 deg. F.) but not at Mora (temperatures below 70 deg. F.). In the Purgatoire River, a thriving population was found where the water temperature was 92 deg. F., on September 6, 1959. In the Arkansas and Pecos rivers and the Rio Grande this subspecies is most abundant below the mountainous parts of the stream-courses, but at elevations higher than 4,000 feet on the plains. _Associated Species_ [Illustration: FIGURE 2. Graphic analysis of head-depth, postorbital length of head and predorsal length of _Hybopsis gracilis_, expressed as thousandths of standard length. Numbers in parenthesis = number of specimens examined from each locality. In each symbol, horizontal line = range, vertical line = mean, open rectangle = one standard deviation on each side of mean, black rectangle = twice the standard error on each side of mean. The dash-lines represent drainage patterns of rivers in which this species occurs. All measurements are of specimens 70 to 100 mm in standard length.] In the Pecos and Arkansas basins, species commonly taken with _H. g. gulonella_ are _Catostomus commersonnii_, _Hybognathus placita_, _Notropis lutrensis lutrensis_, _Notropis stramineus missuriensis_, _Pimophales promelas_, and _Campostoma anomalum plumbeum_. The only spiny-rayed fishes that we have found with _H. g. gulonella_ are _Lepomis cyanellus_ and _L. humilis_, both of which are scarce. Associates of _H. g. gracilis_ include the same species, plus other ostariophysan fishes such as species of _Carpiodes_, _Ictiobus_, and silt-adapted species of _Hybopsis_ and _Notropis_.