We have analyzed the transcription and coding unit of the chicken thymidine kinase (tk) gene. We have constructed a library of mutant chicken tk genes by the in vitro linker insertion method of Heffron et al. A total of 125 mutations within a 3.0 kbp HindIII fragment containing the gene were isolated and mapped. The effect of each mutation upon the thymidine kinase gene was determined by measurement of the transfection efficiency in mouse Ltk- cells. The chicken tk mRNA is about 2 kb and polyadenylated. The direction of transcription was also determined. From these results, we propose a structure for the gene in which at least three small introns separate the amino acid coding region into at least four segments.