AbstractThis paper reports a rather remarkable phenomenon, which we have termed “infantilization.” Young acaras, 7 to 17 days postspawning age and normally freely swimming, will resume their nesting phase when placed with foster parents' brooding embryos or wrigglers. The foster parents do not eat these alien young when the young assume a behavior pattern similar to that of their own brood, namely, nest‐bound and quiescent.