Trisomy 19 mice show a characteristic pattern of delayed morphologic development when compared with normal littermates during the second half of gestation. Examinations of the external phenotype and sectioned fetuses, and skeletal maturation at later days, suggest that trisomy 19 has little recognizable effect on morphogenesis prior to 10 days. Trisomic conceptuses collected at subsequent 24-hour intervals show a 1-day lag in weight gain and begin to show a progressive and uniform delay in differentiation so that 14- and 15-day animals resemble 13- and 14-day normal fetuses, respectively. There is no further recognizable increase in this delay through term. There was no striking change in numbers of viable trisomic conceptuses collected throughout the period from 10 days to birth. These observations suggest that most trisomies surviving at 10 days will develop to term, with the major effect of the aneuploid genome being to delay the otherwise normal-appearing growth and development of the affected conceptuses. In all cases, the litters were from crosses between mice doubly heterozygous for the (5.19) and (9.19) Robertsonian translocation chromosomes.