UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I) and UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II) are key enzymes in the synthesis of Asn-linked hybrid and complex glycans. We have cloned cDNAs from Caenorhabditis elegans for three genes homologous to mammalian GnT I (designated gly-12, gly-13 and gly-14) and one gene homologous to mammalian GnT II. All four cDNAs encode proteins which have the domain structure typical of previously cloned Golgi-type glycosyltransferases and show enzymatic activity (GnT I and GnT II, respectively) on expression in transgenic worms. We have isolated worm mutants lacking the three GnT I genes by the method of ultraviolet irradiation in the presence of trimethylpsoralen (TMP); null mutants for GnT II have not yet been obtained. The gly-12 and gly-14 mutants as well as the gly-14;gly-12 double mutant displayed wild-type phenotypes indicating that neither gly-12 nor gly-14 is necessary for worm development under standard laboratory conditions. This finding and other data indicate that the GLY-13 protein is the major functional GnT I in C. elegans. The mutation lacking the gly-13 gene is partially lethal and the few survivors display severe morphological and behavioral defects. We have shown that the observed phenotype co-segregates with the gly-13 deletion in genetic mapping experiments although a second mutation near the gly-13 gene cannot as yet be ruled out. Our data indicate that complex and hybrid N-glycans may play critical roles in the morphogenesis of C. elegans, as they have been shown to do in mice and men.