Mycelium growth rate is a quantitative characteristic that exhibits continuous variation. This trait has applied interest, as growth rate is correlated with production yield and increased advantage against competitors. In this work, we studied growth rate variation in the edible basidiomycete Pleurotus ostreatus growing as monokaryotic or dikaryotic mycelium on Eger medium or on wheat straw. Our analysis resulted in identification of several genomic regions (quantitative trait loci [QTLs]) involved in the control of growth rate that can be mapped on the genetic linkage map of this fungus. In some cases monokaryotic and dikaryotic QTLs clustered at the same map position, indicating that there are principal genomic areas responsible for growth rate control. The availability of this linkage map of growth rate QTLs can help in the design of rational strain breeding programs based on genomic information.Pleurotus ostreatus is a commercially important edible mushroom commonly known as oyster mushroom. This fungus is industrially produced as human food (4), for the bioconversion of agricultural and industrial lignocellulose wastes (1, 27), and as a source of enzymes and other chemicals for industrial and medical applications (9, 10, 23). Its life cycle alternates between monokaryotic (haploid) and dikaryotic (dihaploid) mycelial phases (8). Two compatible monokaryotic hyphae fuse and produce a dikaryotic mycelium in which the two parental nuclei remain independent (dikaryon, heterokaryon) throughout vegetative growth. Fruiting occurs under appropriate environmental conditions, and true diploidy occurs only at the basidia, where karyogamy takes place immediately before the onset of the meiosis that produces four uninucleate basidiospores.P. ostreatus can be grown on a wide range of substrates, agricultural subproducts, and industrial wastes (11, 25), although pasteurized straw is the most commonly used substrate. Many pests and diseases can cause important yield losses in P. ostreatus cultures. Currently, one of the most important threats to production of this organism is attack by green molds belonging to the genus Trichoderma (30). Differences in green mold susceptibility among P. ostreatus strains have been related to variations in the growth rate of this basidiomycete (2, 29) Additionally, mycelium growth rate has been found to be positively correlated with fruit body yield (28) and thus is an important characteristic for selection in oyster mushroom breeding programs.The mycelium growth rate exhibits continuous variation and is presumably under the control of a polygenic genetic system. The monokaryotic growth rates of some filamentous fungi have been studied and have been found to be associated with some chromosomal regions (14,19,31). However, no quantitative analyses have been performed to determine exhaustively the number, positions, and effects of the genes involved. Quantitative trait locus (QTL) analysis has been applied widely to commercially important plant crops (22, 32, 33) but is rare in studies of mush...