Summary. The effect of plant density on growth, development, yield and quality of kabocha (buttercup squash) (Cucurbita maxima) was examined during 1992–93, at a field site in Cambridge, Tasmania. Plant densities ranged between 0.5 and 4.7 plants/m2. Marketable and total yields were fitted to a yield–density model. Total yield followed an asymptotic trend, approaching 33 t/ha at 4.7 plants/m2, while marketable yield had a parabolic relationship with density. Marketable yield increased to a maximum of 18 t/ha at 1.1 plants/m2, while declining at higher densities because of increased numbers of undersized fruit. Yield of vine marked and callused fruit did not vary with density, but represented a significant proportion of the total yield at all densities. High plant density reduced vegetative growth per plant due to competition for limited resources; as shown by decreased leaf area, number and length of vines, and plant dry weight. Yield tended to decline at high densities because of fewer female flowers and increased fruit abortion per plant. Plants at low densities had more vegetative growth but decreased yields, as increased abortion of fruit relative to the higher plant densities left only 1–2 large fruit per plant. Economic returns varied with plant density. At high densities, variable costs increased (particularly due to high seed cost) while gross income declined reflecting the relationship between marketable yield and plant density. The gross margin therefore declined at high densities.