Newly synthesized ribosomal proteins (r-proteins) are found associated with ribosomal subunits as soon as 90 min after fertilization of Drosophila melanogaster embryos. This event substantially precedes the blastoderm stage of embryonic development, at which time the synthesis of rRNA begins. At the preblastoderm stage, embryos synthesize many and possibly all of the r-proteins, although only a small subset of these is incorporated onto ribosomes, presumably by addition or exchange onto preexisting ribosomes. After blastoderm formation, all of the newly synthesized r-proteins along with newly synthesized rRNA are incorporated into ribosomal subunits.