Early replication of mouse chromosomes was studied with the BrdU-Hoechst-Giemsa technique. With the help of different Robertsonian translocation chromosomes as markers, chromosomal bands that replicate early were identified. An idiogram of each chromosome was constructed. The array of replicating bands along the chromosomes fits well into the existing nomenclature based on G-banding. The karyotype presented is useful for R-banding studies as well.