STATs (signal transducers and activators of transcription) are transcription factors, which lie at the end of cytokine and growth signal transduction pathways. STATs also play crucial roles in embryogenesis, including those pattern formation processes that require organised morphogenetic cell movement. Thus, it is now of extreme importance to know what genes are regulated by STATs, to understand the molecular mechanism of development. For this purpose, we use the cellular slime mould, Dictyostelium discoideum, which is the simplest eukaryotes having STAT proteins, as a model organism. We tried to identify suppressor genes of STATa, a one of four STAT genes (Dd-STATa~d) of Dictyostelium, using a cDNA overexpression library. Although we identified several revertants during screening, identification of the suppressor has been unsuccessful so far. However, we identified a modifier gene, which encodes cyclic nucleotide phosphodiesterase that alters the phenotype of the statA-null. We also searched for genes, which are positively regulated by STATa by use of in situ hybridization and identified 13 genes. To know the functions of some of the genes, we created knockout mutants and found that a disruption of the SLF308 gene, which encodes a putative extracellular matrix protein, caused delayed aggregation and formation of an aberrant fruiting body with a short frail-looking stalk. Such defects are similar to the phenotype of a statA-null strain. These data suggest that the SLF308 is one of targets for STATa and controls morphogenesis, possibly through an interaction between cell surface and extracellular matrices.