Heterosis (hybrid vigor) is defined as the superior performance of the hybrid cross over the mid-parental value.  For topcross progenies, using a common tester, heterotic level narrows down to the performance of the female parent relative to the hybrid.  Topcross hybrids of maize were evaluated for heterosis in a replicated trial at two sites in one season following standard agronomic practices for Kenyan highlands.   The tester (male parent) was an improved maize population at cycle 10, whilst the female parents were either landraces, improved populations or inbred lines.  Mid-parent heterosis was computed for grain yield (T Ha-1), ear height (meters) and days to 50% anthesis.  Statistically significant differences (p< 0.05) among varieties were observed for all traits, except plant stand.  Heterosis varied among hybrids for the three traits.  Heterosis was positive for grain yield and ear height, but negative for number of days to 50% anthesis.  There was a range in variation of heterosis levels for grain yield, ear height and days-to-anthesis.  For grain yield, high percent heterosis was associated with those topcross hybrids in which the female parent was an inbred line or an ill-adapted landrace, implying that the high value was an artifact of the low yield potential of the landraces or inbred line cross, compared to the hybrid.  Generally, percent heterosis for yield was lower in the hybrids in which the female parent was either a single cross or population well-adapted to the highlands and in this latter case, heterosis was considered a genetic fact.