There are a number of low-abundance small nuclear RNAs (snRNAs) in eukaryotic cells. Many of them have been assigned functions in the biogenesis of cellular RNAs, such as splicing and 3' end processing. Here, we present the sequence of Xenopus U12 snRNA and compare the secondary structures of the low-abundance U11 and U12 with those of the high-abundance U1 and U2, respectively. The data suggest functional parallels between these two pairs of snRNAs in pre-mRNA splicing. Using a highly sensitive method, we have identified several new low-abundance snRNAs from HeLa cells. These include five U7 snRNA variants and six novel snRNAs. One of the six novel RNAs is an Sm snRNA, whereas the rest are not immunoprecipitable by either anti-Sm antibodies or anti-trimethylguanosine antibodies. The discovery of these new RNAs suggests that there may be yet more low-abundance snRNAs in the nuclei of eukaryotic cells.