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Serine/arginine-rich splicing factors (SR proteins) are substrates for serine phosphorylation that can regulate SR protein function. We have observed gross changes in SR protein phosphorylation during early development coincident with major zygotic gene activation in the nematode Ascaris lumbricoides. These differences correlate with large-scale changes in SR protein activity in promoting both trans-and cis-splicing. Importantly, inactive early stage extracts can be made splicing competent on addition of later stage SR proteins. These data suggest that changes in SR protein phosphorylation have a role in the activation of pre-mRNA splicing during early development.