Dissociated sponge cells are easily confused with unicellular organisms. This has been an obstacle in the development of sponge-cell lines. We developed a molecular detection method to identify cells of the sponge Dysidea avara in dissociated cell cultures. The 18S ribosomal RNA gene from a Dysidea avara specimen was sequenced and compared to eukaryotic 18S rDNA sequences picked up from a proliferating cell culture that originated from a dissociated Dysidea avara specimen. Our method proved unambiguously that this was not a sponge-cell culture. Therefore, it provides a valuable tool for further research on sponge-cell cultures.