Genetic variation within and among 10 California populations of Ophiostoma clavigerum (Robinson-Jeffrey & Davidson) Harrington associated with the mycangia of Dendroctonus jeffreyi Hopkins was assessed using allozymes. Gene diversity was uniformly low across all populations, averaging 1.4%. Only 2 of 19 gene loci assayed were polymorphic. The three most southerly populations (two from the San Bernardino Mountains in southern California and one from the southern tip of the Sierra Nevada Mountains) were the most differentiated; however, all populations exhibited very low genetic distance. Several factors alone, or in combination could be responsible for the low genetic diversity observed in these populations including (i) a rarity of sexual recombination, (ii) a history involving genetic bottlenecks, and (iii) selection for a mutualistic association with the host beetle.