Activin A, the Inhba gene product, is produced by fetal testis interstitial cells and acts on Sertoli cells to regulate cord development1. In Inhba-/- testes, fetal Sertoli cell proliferation is reduced, yet some gonocytes proliferate longer than normal, yielding an imbalance between Sertoli cell and gonocyte numbers at birth2. The present study investigates how activin influences fetal testis development by examining gene expression in mouse fetal testes with altered activin bioactivity levels. Approaches were: (1) RNAseq to measure transcripts in isolated somatic cells (interstitial, endothelial and Sertoli cells; germ cells excluded) from wildtype and Inhba-/- testes at E13.5 and E15.5; (2) comparing these with pre-existing RNAseq data to identify transcripts enriched in interstitial or Sertoli cells3; (3) high-throughput qPCR to measure specific transcripts in fetal testes from Inhba-/-(no activin A) and Inha-/- mice4 (elevated activin bioactivity). Unaltered levels of steroidogenic enzymes (Cyp11a1, Cyp17a1, Hsd3b1) and other Leydig cell-specific transcripts (e.g. Itih5, Itga8) suggested fetal Leydig cell differentiation was unaffected by activin. However selected Sertoli cell transcripts were significantly altered between E13.5-15.5 by activin deficiency; qPCR confirmed these as activin-responsive genes in Inhba-/-and Inha-/- testes. Transcripts positively regulated by activin included Sel1l3 and Vnn1, and extracellular/secreted proteins Masp1, Sfrp4 and Kazald1. Sertoli cell-enriched transcripts negatively regulated included Cldn11, Tthy1 and Erbb3. Importantly, the Sertoli cell-enriched steroidogenic enzymes, Hsd17b1 and Hsd17b3, were positively regulated by activin. Fetal Sertoli cells express these enzymes to produce testosterone5 which influences gonocyte mitotic quiescence6. These outcomes suggest in conditions of activin insufficiency, androgen precursors production is normal in interstitial cells but inhibited within cords. Identification of these activin-responsive genes in fetal Sertoli cells may be central to understanding how disruptions to this signalling pathway lead to adult pathologies, such as hypospermatogenesis or testicular cancer.