The beta1,2-rhamnosyltransferase WsaF is involved in the biosynthesis of a polyrhamnan chain which is attached to the surface-layer protein from Geobacillus stearothermophilus NRS 2004/3a. The enzyme belongs to the large retaining GT4 family. To date, no structure of a rhamnosyltransferase has been published. Recombinant purified native WsaF has been crystallized, resulting in crystals that belonged to space group P2(1)2(1)2(1) with unit-cell parameters a = 50.5, b = 56.1, c = 276.8 A and diffracted to 3.0 A resolution. Selenomethionine-variant WsaF crystallized in space group P2(1) with unit-cell parameters a = 75.9, b = 75.5, c = 78.1 A and diffracted to 2.3 A resolution.