Mourecotelles Toro & Cabezas (Hymenoptera, Apidae, Colletinae) currently includes only nine valid species of cellophane bees found mostly in relatively-dry regions of western South America (Chile, Argentina, Bolivia, and Ecuador). In this paper, we describe and illustrate a new species of the genus – M. braziliensis Ferrari & Melo, sp. nov. – based on individuals of both sexes captured through trap-nesting in an environmental protection area (Araucárias Municipal Natural Park) and in flowers in

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... different localities in southern Brazil. In total, we obtained 16 nests of M. braziliensis, each consisting of two to eleven brood cells arranged horizontally and lined with a cellophane-like substance. Of the 57 adult bees that emerged, 41 were male (mean weight 46.5 mg) and 16 were female (mean weight 58.9 mg), resulting in biased sex and investment ratios of 2.56:1 and 2.02:1, respectively. Both the numbers of provisioned cells and mortality rate were higher for trap nests with the narrowest bore diameter, although the differences in relation to other trap nests were not statistically significant. Pollen of nine different plant families were found in brood cells of M. braziliensis, but the species showed a clear preference for Fabaceae and Polygalaceae. Indeed, some of the specimens were collected while foraging in flowers of an unidentified species of Monnina Ruiz & Pav. (Polygalaceae) growing in swampy areas. The evolutionary and biogeographical implications of our discovery are briefly discussed.