Abstract Summary: Genetic research utilizes a decomposition of trait variances and covariances into genetic and environmental parts. Our software package biMM is a computationally efficient implementation of a bivariate linear mixed model for settings where hundreds of traits have been measured on partially overlapping sets of individuals. Availability and Implementation: Implementation in R freely available at www.iki.fi/mpirinen. Contact: matti.pirinen@helsinki.fi Supplementary information: Supplementary data are available at Bioinformatics online.