Catostomus macrocheilus, a sucker, and Poecilia reticulata, a live-bearer, are subjected to dissimilar mechanical stresses while most of their bony structures are forming. Osteogenesis occurs in the sucker when it is a free-living larva, whereas in the livebearer it occurs intrafollicularly. The first-formed bones ofboth species are those that meet functional demands and are subjected to the greatest stresses. These are the movable bones associated with the acts of respiration and feeding, and some of the axial bones. The cranial roof is the last to calcify. The primary differences in osteogenesis of the two species, such as in the vertebral column and caudal fin, can be correlated with differences in stress. Surface forces on calcifiable tissue perhapsassist in the deposition of calcium salts.