Four beta-tubulin mRNAs in the embryo of the sea urchin Strongylocentrotus purpuratus are transcribed from at least 3 of the 9-12 beta-tubulin genes. A beta 1 tubulin mRNA of 1.8 kb, transcribed from a unique beta 1 gene, is expressed with high specificity in the pluteus ectoderm. Another 1.8-kb mRNA, beta 2, and a 2.5-kb beta 3 mRNA are moderately ectoderm specific. In contrast, a 3.0-kb beta 4 mRNA is highly specific for the endomesoderm tissue fraction. Certain similarities in developmental and tissue-specific expression suggest that these beta-tubulin genes may be related in their mode of regulation to counterparts among the genes for actin, another cytoskeletal protein. Measurements of absolute amounts revealed a distinct developmental profile for each beta-tubulin mRNA. An increase in the total amount of beta-tubulin mRNA in the early blastula was correlated with an increase in transcription rate per nucleus; whereas, later in the mesenchyme blastula stage, the beta-tubulin mRNA level decreased sharply as the rate of beta-tubulin gene transcription on a per embryo basis remained constant. Thus, during development through the blastula stages, there was a switch to a predominantly posttranscriptional regulation of beta-tubulin mRNA expression, probably through a decrease in mRNA stability.