Softshells have pharyngeal respiration and probably are incapacitated by rotenone. _T. ferox_ and the subspecies of _spinifer_ occur in a wide variety of fresh-water habitats; _muticus_ is more nearly restricted to running water (especially in the northern parts of its range) than _spinifer_, and may be less vagile than _spinifer_. _T. ferox_ is more tolerant of marine and brackish waters than are _muticus_ or _spinifer_. Small size and pallid coloration seem correlated with arid environments. The largest species (_ferox_) and the smallest population of _spinifer_ (resembling _muticus_) both occur in the southernmost part of the range of the genus. Diurnal habits include basking on shores or débris in water, floating at the surface, procuring food, and burrowing in shallow and deep water (no observations for _spinifer_ and _muticus_ in deep water). Softshells are principally carnivorous; the food consists mostly of crawfish and insects; there is evidence of cannibalism involving predation on first- and second-year-old turtles. The capture of food is triggered primarily by movement of prey; sight seems to be more important than smell to _Trionyx_ in capturing food. There is no indication of a food preference between species; enlarged crushing surfaces of jaws in some _ferox_ and _asper_ may be an adaptation for feeding on mollusks. Schools of fish are reported to follow softshells, and presumably acquire food that is dislodged by the grubbing and scurrying of the turtles on the bottom. Softshells are wary. They are good swimmers, and travel rapidly on land. The depressed body is an adaptation for burrowing and concealment. Permanent growths of algae do not occur on the dorsal surface of softshells. There is evidence of some nocturnal activity, and a general parallel in habits between trionychids and chelydrids. Softshells sometimes move overland; they move little in aquatic habitats. The normal annual period of activity of _spinifer_ in latitudes 40° to 43° is approximately five months from April into September, depending on the weather; they hibernate under a shallow covering of mud in deep water. The southernmost populations may be active throughout the year.