A field experiment was conducted from 1989 to 1991 on a Fox loamy sand soil to study the effects of harvest date on selected agronomic, physical and chemical characteristics of peppermint (Mentha piperita L.). Plant biomass and essential oil yields increased throughout the season to a maximum in very late August to early September. Menthol, neomenthol, and menthyl acetate concentrations increased in the essential oil with plant development. The amount of menthone and isomenthone was highest in immature plants. The concentrations of menthofuran and pulegone in the essential oil corresponded to the amount of flower bloom in the peppermint. Changes during the season in the concentrations of piperitone, terpinene-4-ol, α-terpineol, 1,8-cineole, limonene, γ-terpinene, terpinolene, α-terpinene, α-pinene, β-pinene, 3-octanol, β-myrcene, Linalool, sabinene, caryophyllene and Germacrene-D were typically quite small. In general, these preliminary results indicate that peppermint can be grown successfully on the coarse-textured soils of Ontario. Key words: Mentha piperita L., peppermint, essential oil, essential oil composition, harvesting date