Many yeasts in the genus Pichia are killers (yeasts that secrete proteins toxic to other yeasts). The genus is subdivided into complexes comprised of species with similar morphologies, physiological abilities, and percentage of (guanine + cytosine) comprising their nuclear DNA. Here, we test the hypothesis that ecological factors influence the types of toxins produced by members of two Pichia complexes (the Pichia kluyveri and Pichia opuntiae complexes). Members of each complex are separated by geography or host range, but all live in decaying cactus tissue. Similarities among killer toxins were investigated by testing strains from each complex in the laboratory for their ability to kill a standard set of 70 yeasts. Principal components analysis demonstrated that the killer phenotype was constant within a species, but not within the complexes. The probability that a toxin would kill a particular yeast depended on some ecological characteristic of the yeasts tested as sensitives (i.e., the region, host plant, or habitat from which the yeasts were collected). We further tested the hypothesis that ecological factors influence killer phenotype by analyzing within—community patterns of killing. Killer strains from two different cacti were tested for their ability to kill a subset of the yeasts actually collected from the same pockets of rotting cactus tissue from which the killer strains were collected or from rot pockets in neighboring cacti. When the yeasts tested for sensitivity were classified a priori by their probability of interacting with each killer strain, it was shown that the sensitivity to killer toxin was correlated with the probability of interaction. Further, the yeast community composition of a rot pocket depended on the presence or absence of a killer strain in that pocket. We demonstrated that killer toxins could change community composition by growing strains of two sensitive yeasts in the presence of a killer or a non—killer strain of P. kluyveri. The killer strain reached a higher cell density than the non—killer strain in the presence of sensitive yeasts. The density of sensitive yeasts grown with a killer strain was lower than that of a strain grown with a non—killer strain of P. kluyveri.