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Pseudodiplorchis americanus has an extremely brief opportunity for transmission, restricted to less than 24 h in the year when the host (Scaphiopus couchii) enters water to breed. This strict annual cycle means that invading worms have 1 year to complete reproductive development. Despite this, a large proportion of the parasite suprapopulation is not prepared for transmission at the time of host breeding. The present study correlates detailed soil temperatures for one field site in S.E. Arizona, spanning 3.5 years, with laboratory data on parasite growth and reproductive development at a series of controlled temperatures. Development is totally inhibited at a 16-degrees-C. Optimal growth and development occurs at 25-degrees-C and is slower at both 20-degrees-C and a diurnal cycle fluctuating between 20 and 34-degrees-C (mean 27-degrees-C) (simulating summer temperatures experienced by host and parasite). The effective period for accumulation of infective stages by P. americanus in any one season varies according to the timing of the summer rains in consecutive years. Between 1985 and 1988, the interval between host breedings varied from 11 to 13 months and the period when temperatures were above 20-degrees-C varied from 4 to 5.5 months per year. Since the first month post-infection (p.i.) consists of pre-reproductive development, first-year worms have only 3-4.5 months to produce infective larvae. In the shortest seasons, these worms may not be able to complete reproductive preparation in time for the first opportunity for transmission. In different parts of the geographical distribution of S. couchii, different cycles of temperature, rainfall and host breeding occur; varying temperature regimes represent an important abiotic control of P. americanus reproductive biology and transmission.