The abundance of flathead declined slightly from 1957 through 1959, counting fish of all sizes. This trend is attributable to a large hatch in 1957; the 1957 year-class strongly dominated the population throughout my study. Natural mortality in that year-class was compensated by increased average size of the individuals (to six inches in autumn, 1958, and 11 inches in autumn, 1959). The numbers of flathead caught at the upper stations on the Neosho and Marais des Cygnes rivers differed from the general trend in that the species was rare in 1957 and increased slightly by 1959. Flathead are most numerous in large streams, and in the drought they probably were almost extirpated from the headwaters. After 1957, continuous flow and increased volume of flow were accompanied by a gradual increase in numbers of flathead in the upstream parts of the two rivers. The species was most abundant at the middle and lower Neosho stations, where 10.5 per cent of all fish shocked in 1957 and 1958 were _P. olivaris_. The habitat of the flathead varied with size of the individuals. Young-of-the-year inhabited swift riffles having rubble bottom; individuals four to 12 inches in total length were distributed throughout the stream; those more than 12 inches in total length were most commonly in pools in association with cover (rocks, or drifts of fallen timber). Male flathead mature at 15 to 18 inches total length, females at 18 to 20 inches. The spawning season in 1959 probably began in early June and extended to mid-July. I attempted to find spawning fish on 19 June and for one month thereafter. On 19 June nine holes were dug into a 75-yard section of a clay bank adjacent to a long, shallow, rubble riffle. A flathead was first found in one of these holes on 22 June, and others were frequently found in this and one other hole until mid-July. Although channel catfish were often found in nearby holes, that species was never present in the two holes used by flatheads. The holes occupied by flathead (as well as those used by channel catfish) characteristically had silt-free gravel bottoms and a ridge of clean gravel across the entrance. A nest containing a flathead and eggs was located on 11 July. In checking the hole I first put my foot into the entrance, then slowly advanced my hand into the hole, feeling along the bottom with my fingers until they entered the open mouth of a large catfish. I backed off slowly and then felt beneath the fish. The fish was directly above the egg-mass, seemingly touching the eggs with its belly. As I touched the front of the egg-mass the fish struck viciously, taking my entire fist into its mouth. It continued striking until I removed my hand from the hole after obtaining a small sample of eggs, which proved to be in an early stage of development (no vascularization evident).