This paper presents the first complete study of the life history, chromosomes, and general habits of the cactus mealybug, Spilococcus cactearum McKenzie. Importantly, it contains the first description of the male to be published. Taken in conjunction with Howard L. McKenzie’s original description in Hilgardia, Vol. 29, No. 15, June 1960, the present work, with its cited references, rounds out a comprehensive record of all the data available thus far. The life stages are described in detail for both sexes, covering their structures and habits. Laboratory rearing experiments in a glasshouse, from egg to adult, are fully recorded, using several constant temperatures in order to determine the optimum range for maximum survival and colonization, as well as the lethal lower and higher temperatures. Chromosome studies for this species are reported for the first time. Reproduction, sex ratio, and mating behavior are described. It was learned that parthenogenesis does not occur; fertilization by the male is essential. One natural enemy was noted: a minute, wasplike, encyrtid parasite that gave effective control. More than 100 acceptable host plants are listed, mostly cacti. The mealybug’s feeding habits are given, as are its reactions to various host species and the effects its feeding produces upon them. This insect is considered to be the most important pest of ornamental cacti. The exact place of its origin is a bit uncertain: originally reported from southeast England, it has been taken in France, Italy, California, and New Mexico. Its decided preference for xerophytes and need for moderate temperatures point to a semiarid, desert-type habitat.