The ability of various structural analogs of the sexual pheromone sirenin to attract male gametes of the aquatic fungus Allomyces macrogynus was determined. Previous studies had shown that several structural analogs and stereoisomers of natural l-sirenin were devoid of activity at physiological concentrations. We now report the discovery of a structural analog that exhibits biological activity indistinguishable from the natural pheromone. The bioassay system used to determine chemotaxis was calibrated using synthetic, racemic sirenin, which exhibited a threshold concentration for gamete attraction at an applied concentration of 10 picomolar. The new synthetic monohydroxy analog of sirenin also had a threshold concentration of 10 picomolar. In the process of developing a new total synthesis of sirenin, a variety of other analogs were prepared and tested. All of these analogs exhibited threshold concentrations at 1 micromolar or higher, although attraction at these higher concentrations still varied according to their structural resemblance to sirenin. Thus, the results of these studies demonstrate that the hydroxymethyl group attached to the six-membered ring of sirenin is not essential for biological activity at physiological concentrations. The studies with other analogs demonstrate that biological activity at any concentration involves a balance between hydrophilic hydroxyl groups and hydrophobic hydrocarbon groups in the structure.