The catalytic subunit of type 1 serine/threonine protein phosphatase (PP1c) was shown to bind trithorax (TRX) in the yeast two‐hybrid system. Interaction between PP1c and TRX was confirmed in vivo by co‐immunoprecipitation from Drosophila extracts. An amino‐terminal fragment of TRX, containing a putative PP1c‐binding motif, was shown to be sufficient for binding to PP1c by in vitro glutathione S‐transferase pull‐down assays using recombinant protein and fly extracts expressing epitope tagged PP1c. Disruption of the PP1c‐binding motif abolished binding, indicating that this motif is necessary for interaction with PP1. On polytene chromosomes, PP1c is found at many discrete bands, which are widely distributed along the chromosomes. Many of the sites that stain strongly for PP1c correspond to sites of TRX, consistent with a physical association of PP1c with chromatin‐bound TRX. Homeotic transformations of haltere to wing in flies mutant for trx are dominantly suppressed by PP1c mutants, indicating that PP1c not only binds TRX, but is a physiologically relevant regulator of TRX function in vivo.