Patterns of sexual and asexual reproduction were investigated in the lichen genus Parmotrema (Parmeliaceae). Most tropical species were found to produce either sexual or asexual reproductive structures, but not both. Temperate species were found to invest in mixed reproductive strategies. Mixed strategies may be selected for, not only because they maintain genetic variability, but also be- cause they may facilitate lichenization in habitats in which suitable algal-host diversity is low. Correlations were also observed between spore size, production of asexual structures and production of perforate apothecia in fertile species of Parmotrema that suggested a conservation of reproductive tissue in species in- vesting in large spores or asexual structures. Thus, there may be trade-offs in- volved in apportioning resources between growth, maintenance and reproduction in Parmotrema species from habitats that vary in environmental predictability. Lichen life history strategies are usually evaluated with regard to modes of reproduc- tion. Most lichens reproduce sexually by production of ascospores. Many lichen groups are also capable of reproducing asexually by production of specialized thallus fragments (isidia and soredia). Bowler and Rundel (1975) considered various modes of reproduction in the lichen order Lecanorales. From their investigations, they concluded that: (1) asexual groups are derived from sexual ancestors; (2) species rarely produce both sexual and asexual structures on the same thallus; and (3) species exhibiting asexual reproductive mechanisms generally exhibit a broader geographic distribution than sexual species. They suggested that the adaptive advantage of asexual strategies in lichens included increased survival of propagules and rapid invasion of new habitats. Williams (1975) made a number of predictions about the natural history characteristics of sexual and asexual species. A sexual strategy is expected when environmental condi- tions are unpredictable or selection is intense, or both. An asexual strategy is expected in predictable environments or when selection is less intense, or both. Since many organisms exhibit both sexual and asexual reproduction under various conditions during their-life histories, apportionment of resources- between the two mechanisms should reflect the degree to which production of genetically variable propagules by a sexual mechanism is successful. As Bowler and Rundel (1975) observed, lichen species seldom invest in both sexual and asexual mechanisms. They suggested that this reflected an energetic investment too costly to maintain with limited resources. However, in some lichen groups, fertile species also produce asexual structures. There has been no attempt to determine the adaptive advantage of mixed reproductive strategies in these lichen groups. Harper et al. (1970) predicted that seed plants exhibiting mixed (sexual and asexual) reproductive strategies would tend to be found in successional environments where the advantages of having both a high and a low intrinsic rate of increase outweigh the increased energetic cost. Joenje and During (1977) found that some colonizing moss species exhibit 007-2745/80/344-350$1.45/0