Summary 1 Phenotypic plasticity was investigated in the Tatochila sterodice species-group of Pierine butterflies in Chile and Argentina. 2 T. s. macrodice, from the Andes of northwestern Argentina, is monophenic like T. xanthodice of the Colombian Andes. 3 T. s. sterodice from the northern Patagonian Andes and the southern Patagonian coast shows a weak phenotypic response to both photoperiod and pupal chilling; this could form a basis for selection for polyphenism. 4 T. vanvolxemii from central Argentina shows a complex interaction of sexlimited polyphenism in males, with some underlying genetic diversity of response, and sex-limited dimorphism in females. The polyphenism is under temperature control, but photoperiodic influence has not been ruled out. 5 T. mercedis from Mediterranean Chile is polyphenic in both sexes. Photoperiod has been ruled out as a factor. Both larval and pupal temperatures may be significant, especially the former. 6 Both T. vanvolxemii and T. mercedis are probably Quaternary derivatives of T. sterodice, which displays antecedents of both polyphenic systems. 7 Polyphenism in the south-temperate Tatochila is convergent to that in Pieris, having arisen via different selective routes, and is recent enough that further evolution is likely in the major features of the system. 8 The genus Tatochila appears to be primitively rather than derivatively monophenic.