A certain group of English verbs, the ergatives, is consistently mis-passivized in Chinese-speakers' interlanguage. Comparison of the ergative construction in Chinese and English shows that they share similar properties. However, this does not seem to facilitate learning of the target English construction. Furthermore, the passive ergatives are not attributable to exemplification in either the native or the second language, but this distinct type of interlanguage has its own peculiar character. Overgeneration of passivized ergative constructions poses a learnability problem: with no available negative evidence, the tendency to overpassivize can not be reduced. Two learnability theories, the subset principle and the uniqueness principle, offer insight and a partial solution: for a subclass of ergative verbs, the passive can be pre-empted by the ergative construction. However, for more recalcitrant cases, there is no such basis for exemption, hence their susceptibility to fossilization. A 22-item bibliography is included. (Author/MSE)