The development, ultrastructure, and genetics of two mutants that form dauer-like larvae have been characterized. Dauer larva morphogenesis is initiated regardless of environmental stimuli, and it is incomplete or abnormal. The resistance to detergent characteristic of normal dauer larvae is not fully achieved, and the mutants are unable to exit from the dauer-like state of developmental arrest. Mutant life span is not extended beyond the three weeks characteristic of the nondauer life cycle, whereas normal dauer larvae can live for several months. Growth of daf-15(m81)IV, the less dauer-like of the two, is nearly arrested at the second (dauer-specific) molt, but feeding is not completely suppressed. Head shape, cuticle, and intestinal ultrastructure are nondauer, whereas sensory structures (amphid and deirid) and excretory gland morphology are intermediate between that of dauer and nondauer stages. The daf-9(e1406)X mutant is dauer-like in head shape, cuticle, and deirid ultrastructure, intermediate in amphid and inner labial neuron morphology, and nondauer or abnormal in the intestine. Also, the daf-9 mutant exhibits abnormalities in the pharyngeal arcade cell processes and pharyngeal g1 gland. Double mutants carrying both daf-9 and daf-15 are more resistant to detergent than either single mutant. Like the single mutants, they cannot complete morphogenesis, and they are unable to exit from the dauer-like stage. Both daf-9 and daf-15 mutations are epistatic to previously described dauer-defective mutations, indicating that these two genes act late in the pathway leading to the dauer larva. The genetic tests and the mutant ultrastructure suggest that the two genes may affect parallel pathways of morphogenesis.