Scirtothrips perseae Nakahara causes significant scarring of avocado fruit and is the most serious pest of avocado in southern California. Because it was described only in 1997, no information that can be used for its control exists, including the relationships between its densities on leaves and fruit and scarring damage on fruit. To determine these relationships, percentages of scarring damage on avocado fruit were regressed against thrips numbers/leaf and numbers/fruit using data collected from three orchards in Ventura and Santa Barbara Counties, CA, from 1997 through 1999. Thrips population increases were positively related to leaf growth flushes. Numbers of first and second instars, total larvae, and adults/leaf were all predictive of scarring on fruit. Peaks of three to five larvae/leaf before and during fruit set predicted 6–15% economic scarring damage on fruit from October through December, whereas means of 0.5–1.5 larvae/small fruit predicted 22–51% scarring. Fruit were most susceptible to damage within a 2-wk period during and shortly after fruit set when they measured 0.53–1.42 cm long. Results suggest S. perseae numbers on leaves during fruit set can be used to predict scarring damage on fruit, and that damage thresholds may be ≤5 larvae/leaf during this time. Because low numbers of larvae on fruit within a 2-wk period can result in high scarring damage, early treatments when thrips are on leaves, just before they move onto fruit, may help prevent damage to fruit.