Cronartium comandrae Peck is a heteroecious native rust which is damaging to several Pinus species in North America. It grows perennially in the living bark of hard pines producing pycniospores and aeciospores, and develops annually on species of Comandra and Geocaulon producing urediospores, teliospores and basidiospores. Studies were carried out to determine the effect of various meteorological and biological environmental factors on the aeciospore aerobiological phase of the rust. The main aspects considered included the factors affecting aeciospore production and release; and the factors affecting aeciospore transport, dispersion, deposition, germination and viability. Aeciospore production occurred from mid May to late August, with the peak period between late May and mid June. An average aecium produced spores for 35-50 days, and individual cankers produced spores for up to 95 days. Much variation occurred from year to year and between trees. Aecial production was interfered with by the activity of rodents, insects and other fungi on the canker and through resinosis, which probably accounted for a 50-55% reduction of the potential aeciospore production in any year. Fresh rodent damage was recorded on 40-52% of the cankers at about 20 locations in the years I966 to I968, insect damage on 39-46%, Tuberculina and Cladosporium infection on 17-33%, and resinosis on 67-71%. Rodent damage was mainly caused by squirrels. Some 64 micro-floral organisms were isolated from the cankers and spores. Tuberculina maxima was mainly responsible for killing the infected canker bark and greatly reduced the production of aeciospores. Penicillium spp. and an undescribed Cladosporium were also prominent on the cankers but played a lesser role in reducing production. One hundred and seventeen species of insects, mites and spiders were associated with the rust canker. Epuraea obliquus, an unidentified cecidomyiidae and Paracacoxenus guttatus were true mycetobionts.