An experimental investigation of primary breakup of nonturbulent round liquid jets in gas crossflows is described. pulsed shadowgraph measurements of jet primary breakup regimes, jet deformation properties, time of onset of primary breakup, and liquid column and liquid surface disturbance wavelengths were obtained for air crossflows at normal temperature and pressure. The test range included crossflow Weber numbers of 0-200 liquid jet Weber numbers of 0-1,600,000 liquid jet Reynolds numbers of 0-800,000 liquid/gas momentum ratios of 100-8000 liquid/gas density ratios of 700-1100 and Ohnesorge numbers of 0.00006-03. The results suggest qualitative similarities between the primary breakup of nonturbulent round liquid jets in crossflows and the secondary breakup of drops, with relatively little effect of the liquid/gas momentum ratio over the present test range. Effects of liquid viscosity were also small for Ohnesorge numbers less than 0.1, but larger Ohnesorge numbers modified primary breakup because unusually long ligaments developed that also are seen during the secondary breakup of drops.