Serum alpha-L-fucosidase was assayed in 422 individuals. Extremely low levels of enzyme activity were found in 26 individuals. This variant enzyme has increased heat lability. The remaining samples exhibited a bimodal frequency distribution. Family studies suggest that the variant is inherited as a Mendelian recessive character. The variant leukocyte enzyme exhibits normal activity yet increased heat lability. Serum and leukocyte samples from individuals who do not possess the low activity variant are heterogeneous with respect to the heat lability of alpha-L-fucosidase. This common polymorphism may be useful for mapping purposes in family studies.