Pycnial nectar of one mating type is known to induce cap formation on pycniospores of opposite mating type within several species of Puccinia and Uromyces. To learn if caps are induced by nectar transfers between species, we used interspecific pairings involving six species of Puccinia and three of Uromyces. Overall, caps were induced in 14 pairings between different species involving all tested species, except Puccinia helianthi which has no intraspecific cap induction. Nectar (with pycniospores) exchanged in reciprocal transfers between individual pycnial clusters of two different species gave pycniospore caps in nine of 16 cluster pairings, comparable to rates within species. Spore-free nectar combined from five or more pycnial clusters of one species (to ensure that nectar of two mating types was present) usually induced caps in pycniospores from single pycnial clusters of a second species. This occurred in all tested pairings of species except pairings involving P. helianthi. In experiments with pycniospore-free nectar of one capping type specificity from P. recondita, caps were induced in about 50% of pycnial clusters of unknown capping type from P. triticina or P. hordei and only in pycnial clusters of one capping type from P. triticina or P. reichertii in experiments in which type within species was determined. Coupled with the fact that capping type specificity and mating types are coincident within species, the results indicate that mating type-specific induction of pycniospore caps by nectar extends across species boundaries. Although aecia were never produced in interspecific pairings, cap induction occurred as it does in intraspecific pairings where it precedes aecium formation in species exhibiting the capping phenomenon.