The aim of this study was to measure the effects of an increase in the density of the Colorado potato beetle,Leptinotarsa decemlineata(Say), on this insect. Two groups of 20 potato plants,Solanum tuberosumL., ‘Superior’, were subjected to defoliation by the Colorado potato beetle. The first group was infested by 15 larvae per plant and the second by 40 larvae per plant. Measurements of adult body weights were taken, as well as of the quantity of lipid reserves of adults collected upon emergence. Insects originating from plants infested by 15 larvae were significantly heavier and had accumulated significantly more lipid reserves than adults originating from plants infested by 40 larvae. The percentage of lipids over body weight was significantly lower at a density of 15 larvae per plan than at a density of 40 larvae per plant. The effects measured in the 40 larvae per plant group are the result of intraspecific competition phenomena.