Grain yield loss assessment trials were conducted between 1997 and 1999 in Samaru, Nigeria, to assess losses in millet grain caused by the stemboring crambid moth, Coniesta ignefusalis Hampson in four millet-based cropping systems. Deadhearts were scored at 35 and 45 days after planting (DAP) and bored and unbored stems at 95 DAP (harvest). Grain yield loss 35 DAP in 1997 was 265.56 kg/ha of monocropped millet and this was significantly different from the 146.29 kg/ha if millet was intercropped with cowpea. At 45 DAP in 1998, the loss of monocropped millet was 50.48 kg/ha and it was significantly higher than the 13.44 and 10.00 kg/ha of millet intercropped with groundnut or cowpea, respectively. Losses were non-significant under all cropping systems at 45 DAP in 1997 and 35 DAP in 1998. Assessing for yield losses at harvest (95 DAP) in the 3 years was difficult because in some cases, bored stems out-yielded unbored ones. However, the mean grain yields of stems bored by the stemborer under millet monocrop in 1997 (3881.37 kg/ha) and 1998 (4308.15 kg/ha) were significantly higher than those under millet intercrops in both years. Similarly, mean grain yields of unbored stems under millet monocrop were 4327.11 and 5128.78 kg/ha in 1998 and 1999, respectively, and these were also significantly different from those of unbored stems under all millet intercrops in both years. The mean grain yields of all unbored plants were non-significant in 1997. Using linear regression analysis, a significant and positive correlation was established between yield loss and larval population under millet monocrop in 1997 (r = 0.871). A similar relationship was observed between yield loss and percentage plant infestation (deadhearts) under all cropping systems in 1997 and under millet monocrop (r = 0.994) and millet–cowpea (r = 0.8487) intercrops in 1998. Percentage plant infestation rather than larval numbers was found to be the better indicator of yield loss in millet.