Abstract Tekle, Y. I., Raikova, O. I. and Jondelius, U. 2006. A new viviparous acoel Childia vivipara sp. nov. with observations on the developing embryos, sperm ultrastructure, body wall and stylet musculatures. —Acta Zoologica (Stockholm) 87: 121–130 A free-living viviparous acoel, Childia vivipara sp. nov., from the Gullmar fjord of the Swedish coast is described. The new species is assigned to the taxon Childia based on histological, ultrastructural and molecular sequence similarities. All available molecular markers (18S rRNA, 28S rRNA and histone H3) and several morphological characters, obtained using transmission electron microscopy and confocal scanning laser microscopy of whole mount specimen stained with TRITC-labelled phalloidin, support the placement of C. vivipara in the taxon Childia. Childia vivipara and other Childia species share the following morphological synapomorphies: well-developed copulatory organs built of tightly packed stylet needles, proximal part of the stylet inserted into the seminal vesicle, reversed body-wall musculature, absence of ventral diagonal muscles, presence of dorsal diagonal muscles, and presence of ventral straight longitudinal muscles between frontal pore and mouth, 9 + 1 sperm axoneme structure, six distal sperm cytoplasmic microtubules, and extensive overlap of axonemes and nucleus. The new species can be easily distinguished from other Childia species by its viviparous mode of reproduction and single curved stylet. Observations on late embryonic development based on the oldest developing embryos are discussed.