The ace-1 gene, which encodes acetylcholinesterase of class A, has been cloned and sequenced in C. briggsae and compared to its homologue in C. elegans. Both genes present an open reading frame of 1860 nucleotides. The percentages of identity are 80% and 95% at the nucleotide and aminoacid levels respectively. All residues characteristic of an acetylcholinesterase are found in conserved positions in C. briggsae ACE-1. The deduced C-terminus is hydrophilic, thus resembling the catalytic peptide T of vertebrate cholinesterases. Codon usage in both ace-1 genes appears to be lowly biased. This may indicate that these genes are lowly expressed. The splicing sites of the eight introns of ace-1 in C. elegans are conserved in C. briggsae, but introns are shorter in C. briggsae. No homology was found between intronic sequences in both species, except for the consensus border sequences.