A temperature-sensitive lethal allele of suppressor of forked, l(1)su(f)(ts67g) (ts67), has been discovered and characterized as follows: Flies which are hemizygous for ts67 live at 18 degrees and 25 degrees but die at 30 degrees primarily as larvae. The temperature-sensitive period for ts67 homozygotes or hemizygotes begins in second instar and ends at pupation. ts67 is lethal at 30 degrees when heterozygous with suppressor of forked (su(f)), a deficiency for suppressor of forked (su(f)(-)), and a non-conditional lethal allele of suppressor of forked (3DES). It is viable at 30 degrees when heterozygous with the wild-type allele of suppressor of forked. At 25 degrees but not at 18 degrees forked bristles are suppressed in flies of the following genotypes: f(s)ts67/Y, f(s)ts67/f(s)ts67, f(s)ts67/f(s)su(f), f(u)ts67/f(s)3DES, f(u)ts67/f(s)su(f)(-), f(u)ts67/f(s)su(f). There is some suppression of forked bristles at 25 degrees in the heterozygote, f(s)ts67/f(s)+(su(f)). The forked bristle phenotype is not suppressed at either temperature in flies of the genotypes f(u)ts67/Y, f(u)ts67/f(u)ts67/ (f(s) and f(u) indicating suppressible and unsuppressible alleles of forked). The temperature-sensitive period for suppression of forked bristles begins at pupation and extends through the period of bristle synthesis. The deficiency phenotype (bristles reduced in size or absent, wing wrinkled or blistered, eyes rough) typical of flies of the genotype f(s)su(f)/f(s)su(f)(-) at 18 degrees and 25 degrees , is exhibited by flies of the genotypes f(s)ts67/f(s)su(f)(-) at 25 degrees and f(u)ts67/f(s)su(f) at 29 degrees . An allele of lozenge (lz(1)) which can be suppressed by su(f) is suppressed at 25 degrees but not at 18 degrees in lz(1)ts67/Y males. ts67 homozygous females are fertile at 25 degrees but sterile at 30 degrees . The hypothesis is discussed that the su(f) locus codes for a ribosomal protein and that suppression and enhancement are affected by mutations at the locus by mutant ribosome-induced misreading. The possibility is presented that ts67 may be used to determine the translation time in development of any gene.