This paper uses Esperanto---a constructed language with transparent morphology but rich semantic-pragmatic components---to study antonymy and polarity. We investigate the distribution of the Esperanto antonymy morpheme "mal-" (as in, for instance, "mal-alta": "antonym-tall", "short") in a 4.3 million-word corpus, Tekstaro, and use it as an empirical basis to assess different theories of negative antonyms. Our methodology consists in investigating the extent to which the antonymy morpheme "mal-", which we take to denote negative polarity, bears the linguistic features predicted by traditional linguistic tests (such as incompatibility with measure and ratio phrases and low likelihood of nominalisation).