A. Grain sorghum - 1966 and 1967. The effects of plant spacing on yield and yield components of RS610 hybrid grain sorghum were studied in 1966 and 1967 at Urbana, Illinois. Grain yield/plant was, as expected, reduced in all experiment as plant population was increased. Plants at low populations were able to compensate somewhat for low stand densities through production of tillers. More tillers were produced in narrow than in wide rows in one of the two experiments conducted in 1966. A generally higher/seed at low than at high plant population increased yield/plant at low population. In spite of tillering which increased the number of seeds/plant and production of heavier seeds by plants at low stand densities, yield/m2 increased as plant population increased to between 25-48 plants/m2, depending on the experiment. While RS610 hybrid grain sorghum has been noted for its high capacity to tiller, the results of these experiement indicate that it lacks the potential to produce a sufficient number of tillers/plant and/or seeds/panicle to offset low planting rates at Urbana, Illinois. Per plant yield from a study utilizing a systematic planting design, in which observations were single systematically arranged plants, were similar to those obtained in a large plot experiment. Small per plant difference were magnified, however, when they were multiplied by the number of plants/m2 so that predicted yield/m2 from the systematic study did not correspond closely to those obtained in the large plot experiment. (...).