Adenylyl cyclase activity was detected in a mixed-membrane fraction from dormant spores of Phycomyces blakesleeanus. This enzymatic activity increased linearly as a function of protein concentration up to 300 μg of protein 100 μl−1 and 20 min incubation at 25°C. It used Mn2+ or Mg2+ indiscriminately as a cofactor, and the addition of both cations together did not have a synergistic effect. The crude enzyme showed a Kmapp for ATP of 0·25 mM, when measured in the presence of Mg2+. It was stable for 48 h at −20°, losing 25% of its activity after 72 h. The addition of 10 μM GTP to the enzymatic assay stimulated the adenylyl cyclase, whereas higher concentrations (500 μM) inhibited it. Cholera toxin and 25 μM forskolin caused a two-fold stimulation of the enzymatic activity. The calcium-calmodulin complex stimulated activity two-fold; this stimulation was inhibited by the anti-calmodulin drug trifluoperazine. The enzyme could not be solubilized by NaCl, but was partially solubilized with non-ionic detergents, indicating that the enzyme is an integral membrane protein. The detergent-solubilized enzyme only used Mg2+ as a divalent cation and was also stimulated by calcium-calmodulin, low concentrations of GTP, cholera toxin and forskolin, but was extremely unstable. These results suggest that the adenylyl cyclase present in dormant spores of Phycomyces blakesleeanus is an integral Type I-like membrane enzyme.