Chimeric hydra were prepared by recombining epithelial and interstitial cells between 3 strains of hydra of different sizes (maxi, normal, and mini strains). The resulting chimeras generally resembled the epithelial cell parent more than the interstitial cell parent in size, budding rate, tentacle number, and form. This suggests that epithelial cells normally exert considerable influence over hydra morphogenesis. However, the chimeras show some differences ascribable to interstitial cell origin. Furthermore, the 3 original strains, when deprived of interstitial cells, lose their distinguishing size differences. Thus both epithelial and interstitial cells (or interstitial cell derivatives) mutually participate in hydra's development.