Populations where three different lethals are segregating as alleles have been analysed for the conditions of equilibrium and for the trends during elimination of one allele. Early and late selection as well as sexdependent and sexindependent selection has been taken into consideration.Cage populations of Drosophila melanogaster with different fourth chromosome lethals have been followed and compared to the theoretical model. When two marker chromosomes (ciDpol and spaCat) and one out of four recessive lethal chromosomes l(4)5, 1(4)8, 1(4)10 or l(4)14 were used, the same marker chromosome (ciDpol) became extinct in all populations. Early and late selective values which were obtained directly from the populations were compared to estimates of fitness components obtained in specially designed experiments of viability, developmental rate, mating ability and fecundity. When two out of the four recessive lethals and the marker chromosome, spaCat , were combined in new populations, all populations attained equilibrium withôut extinction. A correlation was found between the time of death of the lethal homozygotes and the equilibrium genotype frequencies. Overall selective values at equilibrium were estimated. / digitalisering@umu