The cellular phenotypes are described for 28 temperature-sensitive embryonic arrest mutants in 25 genes in the nematodeCaenorhabditis elegans. Cell lineages, and cellular and subcellular properties at the non-permissive temperature (26° C) have been studied by direct observation of individual cells in living embryos using Nomarski microscopy and high-resolution video recordings. The sequence, direction and time of division and the position of the individual cells have been compared to wild-type development up to at least the 100-cell stage (or earlier stage of arrest). Defects are related to the previously reported arrest stage, temperature-sensitive period, and to maternal effects. Most maternal mutants display defects in zygote formation. These include absence of pronuclear fusion or of polar bodies, absence or abnormal position of pseudocleavage or of the first division cleavage, anomalous cytoplasmic streaming, eggshell defects, abnormal cytoplasmic yolk granules, extra(pro)nuclei, endomitosis or arrest at the one-cell stage. During embryogenesis, many mutants show cellular and/or morphological abnormalities, including pseudopodia, blastocoel malformation, prolonged mitosis and membrane reformation, ill-defined membranes, segregation of extra nuclei, and cytoplasmic "plaques" at division. Most mutants display defects in cell lineage features, i.e. slow cell division rate, abnormal division sequence or direction. Three mutants show premature germ-line cell division, one of these also having a supernumerary germ-line division. Nine mutants show defects in the intestinal cell lineage, i.e. in division direction, in timing relative to gastrulation or in intestine anatomy. This survey of the cellular properties of the mutants provdes a basis for a more detailed, e.g. ultrastructural and molecular, study.