The demosponge Oscarella lobularis (Schmidt, 1862) has been reported from various localities throughout the world and is abundant in many areas. It is considered to be morphologically very variable, and morphs of different colour or texture can be found living sympatrically. Samples of a soft violet morph and of cartilaginous blue,yellow and green morphs were collected from the Mediterranean Sea, near Marseilles. The genetic divergence of the four morphs was estimated by enzyme electrophoresis, and cytological differences were studied using light and transmission electron microscopy. The cartilaginous morphs were genetically practically identical, but the soft violet morph was diagnostically different from the sympatric samples of the blue and green morphs and from the allopatric yellow morph at most of the genetic loci studied (genetic identity, I ~ 0'25). The high genetic divergence was associated with consistent differences in morphology and cytology between the violet morph and the other colour morphs. This is considered to justify a specific distinction. It is concluded that the name Oscarella lobularis (Schmidt, 1862) should be applied to the soft violet morph only, whilst the other, cartilaginous colour
morphs are referred to Oscarella tuberculata (Schmidt, 1868).