The dihydropyrrolizine pheromones, hydroxydanaidal and danaidal, were identified from the scent organs of malePhragmatobia fuliginosa(L.) andPyrrharctia isabella(J.E. Smith). Qualitative and quantitative GLC analyses were conducted on ca. 80 field-collected males of each species. The total pheromone titer was distributed bimodally in each species with most males having either a small amount (<10 ng) of pheromone or a large amount (1–10 μg inPyrrharctiaand 0.3–3 μg inPhragmatobia).Pyrrharctiamales in the 1- to 10-μg range had a predominance of hydroxydanaidal, with little if any danaidal. MostPhragmatobiamales in the 0.3- to 3-μg range had danaidal with little if any hydroxydanaidal. These compounds elicited a courtship response in sexually receptive females of both species. A bioassay based on this response was used to measure the thresholds of female response to these compounds.Pyrrharctiafemales were more sensitive to (R)-(−)-hydroxydanaidal than to danaidal.Phragmatobiafemales were more sensitive to danaidal then to (R)-(−)-hydr