Regulators responsible for the pervasive, nonsex-specific alternative pre-mRNA splicing characteristic of metazoans are almost entirely unknown or uncertain. We describe here a novel family of splicing regulators present throughout metazoans. Specifically, we analyze two nematode (Caenorhabditis elegans) genes. One, CeSWAP, is a cognate of the suppressor-of-white-apricot (DmSWAP) splicing regulator from the arthropod Drosophila. Our results define the ancient, conserved SWAP protein family whose members share a colinearly arrayed series of novel sequence motifs. Further, we describe evidence that the CeSWAP protein autoregulates its levels by feedback control of splicing of its own pre-mRNA analogously to the DmSWAP protein and as expected of a splicing regulator. The second nematode gene, Ceprp21, encodes an abundant nuclear cognate of the constitutive yeast splicing protein, prp21, on the basis of several lines of evidence. Our analysis defines prp21 as a second novel, ancient protein family. One of the motifs conserved in prp21 proteins--designated surp--is shared with SWAP proteins. Several lines of evidence indicate that both new families of surp-containing proteins act at the same (or very similar) step in early prespliceosome assembly. We discuss implications of our results for regulated metazoan pre-mRNA splicing.