The chemical study of the sponge Mycale cecilia has led to the isolation of 14 new pyrrole-containing metabolites. Mycalazals 3–13 are pyrrole-2-carbaldehydes possessing at C-5 hydrocarbon side chains of different length and/or number of unsaturations. Mycalenitriles 1–3 are 5-cyanoalkylpyrrole-2-carbaldehydes. The structures of the new compounds were established mainly by NMR and MS spectroscopic analysis. The location of the double bond in mycalazal-4, -8, and -11 was determined by MS analysis of the corresponding bis(methylthio) derivatives. Mycalazals have shown activity as growth inhibitors of several tumor cell lines, in particular the LNcaP cell line, being mycalazal-8 the most active metabolite.