SummaryGenetic linkage between the equine loci for phosphohexose isomerase (PHI) and serum Xk protein was demonstrated by means of segregation data from three sire families. The recombination frequency was estimated from pooled data to be 0.23 ± 0.02; a significant heterogeneity between sires for estimates of the recombination frequency was observed. No indication of linkage was detected betweenXkand 14 other blood marker loci. Linkage between theXklocus and the locus for soluble malic enzyme (ME1) has recently been reported in horses. An equine linkage group designated LG IV comprising the three lociME1, PH1, andXkhas thus been established. The possibility that the linkage betweenPH1andXkis homologous to the linkage between the loci for PHI and a serum postalbumin (PO‐2) in pigs was discuss