Sponges (phylum Porifera) are simple metazoans for which no molecular information on gametogenesis and larval development is available. To support the current study, it was confirmed by histology that oocytes and larvae were produced by the demosponge Suberites domuncula. Three genes/expressed products from S. domuncula whose expression correlated with sexual reproduction were identified and characterized (they are used here as marker genes): i) a receptor tyrosine kinase (RTK) with sequence similarity in the tyrosine kinase domain to fibroblast growth factor receptors; ii) the sex-determining protein FEM1 and iii) the sperm associated antigen (SAA) of triploblasts. Antibodies against the extracellular domain of the RTK specifically stained oocytes and larvae in S. domuncula tissue sections. Induction of these three genes was successful at elevated temperature, a factor which also promotes natural gametogenesis. In situ hybridization analyses revealed that FEM1 and SAA were expressed in those areas in which gametogenesis begins. Our results indicate that genes which play a role in sex determination may be present in Porifera.