The effect of competition among same-aged larvae of the vegetable leafminerLiriomyza sativaeBlanchard was investigated in laboratory studies over a range of densities from 0.1 to 2.9 larvae per cm2of lima bean primary leaf area. Both exploitative and interference competition (cannibalism) occurred among larvae. Cannibalism was observed primarily between first or second instal's, and was not density-dependent at<1.0 first instal's per cm2. Percentage larval survival (y) was ∼90% at densities of<1.0 larvae per cm2but declined significantly as the density of first instal's(x)increased(y= −20.0x + 115.6, t = −9.0, df = 47, P<0.0001, r2= 0.64). Among leaves with densities<1.0 first instal's per em, there was no significant relationship between larval weight (yin micrograms) and density (x) (t= −1.4, df = 194,P= 0.18), although the relationship was significant when leaves across the entire range of densities were included (y= −108.5x + 702.3,t= −19.1, df = 1603, P<0.0001). Exploitative competition occurred between third instal's at higher densities. Larval weights varied considerably among individuals utilizing the same leaf and density explained only 18% of the variability in weight. Larvae from leaves with a mean first-instal' density of 2.8 larvae per cm2weighed, on the average, 37% less than those emerging from leaves with 0.3 larvae per cm2. Percentage pupal survival (y) decreased significantly with increasing density (x) (y= −10.9x+ 102.5,t= −4.3, df = 33,P= 0.0001,r2= 0.37). The area of individual leaf mines measured 0.99 ± 0.12 cm2($\bar x$± SEM). There were no apparent seasonal trends in area of leaf mines.