We encountered the problem of calculating the approximate surface area of a citrus tree in the process of developing a plot design for a mature citrus grove with unequal tree size. The objective of our experiment was to observe the impact of alternative cultural and pest management practices on fruit yield. Fruit yield for the year prior to experimental maniupulation is one parameter that has been used in selecting replications in such experiments (Parker and Batchelor 1932, Parker 1942, T. W. Embleton, personal communication). A second useful measurement is trunk diameter at a specified distance above the bud union (Embleton and Jones 1973). In our experiment, neither of these parameters were wholly satisfactory. Fruit yield in the prior year was somewhat variable due to low fruit set. In addition, a winter frost which had occurred shortly after planting resulted in considerable trunk deformation, dieback, and regrowth such that trunk diameter measurements were variable for similar sized trees and thus suspect as indicators of tree size and production. Citrus tree canopies are quite dense and the majority of foliar growth and fruit production occurs near the exterior surface of the tree. Because of the dense tree canopy, we hypothesized that the external canopy surface area of the tree would be proportional to the effective leaf surface area of the tree with regards to light interception and photosynthesis. In fact, Jahn (1979) showed that the canopy area ratio (leaf area/external canopy surface area) was fairly constant for 9 varieties of citrus and that this ratio was linearly related to penetration of photosynthetically active radiation. Thus, we considered canopy surface area to be a reasonable covariate (in addition to prior year fruit yield) upon which to base our experimental design. Derivation of a formula for canopy surface area depends upon tree shape. A simple solution is obtained when tree shape is assumed to approximate that of a half sphere. Canopy surface area (SA) is given by (1) assuming tree height (a) equal to canopy base across-row radius (b) and within-row radius (c) (see Fig. 1). Allen and McCoy (1979) assumed a spherical tree shape when modeling the thermal environment of the citrus rust mite on citrus fruit. The use of a more complex shape, in their case, would have led to somewhat intractable mathematics. In many cases, however, tree height and radius are dissimilar, tree height often approaches three diameter (twice the radius).