This paper examines trends in yields of grain corn, barley, and winter wheat in Ontario between 1888 and 1995. An economic model of crop yields is estimated for each of these crops for the entire time period from 1888 to 1995 and for the post‐1945 period. The goal of the empirical analysis is to separate the effects of prices, weather, and land quality from the effect of technological change, represented as a time trend, on yields. Yields of the three crops considered in this study remained roughly constant until about the end of World War II. In the post‐war era, all three crops exhibited clear upward trends. Regression models with a quadratic time trend fitted to the post‐1945 data indicate that yields of these crops have been increasing at a decreasing rate. The estimation results for this most recent 50‐year period indicate, however, that yields have not yet reached a plateau and that they will not do so for some time.