During the development of Dictyostelium discoideum, several thousand new mRNA species appear in the cytoplasm after the cells have formed stable aggregates. Here we show that six of these late mRNAs, corresponding to six clones randomly chosen from a genomic library, are synthesized from the very beginning of development at a rate comparable to that observed late in development but that transcripts do not accumulate until after aggregation. The early- and late-synthesized mRNAs are identical in size and compete with each other for hybridization to the genomic clones. The early-synthesized mRNAs do not accumulate in the cytoplasm in the preaggregation stage because they are very unstable. Their stability, estimated from the kinetics of incorporation during continuous labeling with (32)P, increases by perhaps an order of magnitude in the postaggregation stage. We conclude that mRNA stabilization is the major controlling factor of the expression of these genes.