Abstract After selection for three generations, thick- and thin-shelled lines of Single Comb White Leghorns (SCWL) were developed which produced eggs having means of 10.7 and 7.9% shell, respectively, at twleve months of age. The average time interval between ovipositions of the first and second eggs in a laying cycle at 16 months of age was 27 hr 20 min for the thick-shelled line and 27 hr 35 min for the thin-shelled line, respectively. Through digital palpation, in utero times were determined at 17 months of age. Eggs from birds of the thick-shelled line spent an average of 20 hr 5 min and those from birds of the thin-shelled line spent 19 hr 38 min in the shell gland. The percent shell was 9.7 and 7.0 for the thick- and thin-shelled lines, respectively. It is concluded that the two lines differ in shell quality because of differences in the rate of shell deposition.