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<p>Abstract</p> <p>Background</p> <p>The nematode <it>Caenorhabditis elegans </it>is a major laboratory model in biology. Only ten <it>Caenorhabditis </it>species were available in culture at the onset of this study. Many of them, like <it>C. elegans</it>, were mostly isolated from artificial compost heaps, and their more natural habitat was unknown.</p> <p>Results</p> <p><it>Caenorhabditis </it>nematodes were found to be proliferating in rotten fruits, flowers and stems. By collecting a large worldwide set of such samples, 16 new <it>Caenorhabditis </it>species were discovered. We performed mating tests to establish biological species status and found some instances of semi-fertile or sterile hybrid progeny. We established barcodes for all species using ITS2 rDNA sequences. By obtaining sequence data for two rRNA and nine protein-coding genes, we determined the likely phylogenetic relationships among the 26 species in culture. The new species are part of two well-resolved sister clades that we call the <it>Elegans </it>super-group and the <it>Drosophilae </it>super-group. We further scored phenotypic characters such as reproductive mode, mating behavior and male tail morphology, and discuss their congruence with the phylogeny. A small space between rays 2 and 3 evolved once in the stem species of the <it>Elegans </it>super-group; a narrow fan and spiral copulation evolved once in the stem species of <it>C. angaria</it>, <it>C</it>. sp. 8 and <it>C</it>. sp. 12. Several other character changes occurred convergently. For example, hermaphroditism evolved three times independently in <it>C. elegans</it>, <it>C. briggsae </it>and <it>C</it>. sp. 11. Several species can co-occur in the same location or even the same fruit. At the global level, some species have a cosmopolitan distribution: <it>C. briggsae </it>is particularly widespread, while <it>C. elegans </it>and <it>C. remanei </it>are found mostly or exclusively in temperate regions, and <it>C. brenneri </it>and <it>C</it>. sp. 11 exclusively in tropical zones. Other species have limited distributions, for example <it>C</it>. sp. 5 appears to be restricted to China, <it>C</it>. sp. 7 to West Africa and <it>C</it>. sp. 8 to the Eastern United States.</p> <p>Conclusions</p> <p><it>Caenorhabditis </it>are "fruit worms", not soil nematodes. The 16 new species provide a resource and their phylogeny offers a framework for further studies into the evolution of genomic and phenotypic characters.</p>