Melanism in the oriental armyworm Mythimna separata (Walker) is characterized by a black color in adults instead of the normal body color of off-white. Because inheritance of melanism in this moth follows Mendel’s law, with the melanic allele (t) recessive to the normal allele (T) at a single autosomal locus, a backcrossing procedure was used to develop near isogenic lines (NILs) for melanic (NILs-tt) and typical forms (NILs-TT) of this moth. Melanic males (tt) and typical females (TT) were crossed in the laboratory, their progeny (F₁) were interbred, and F₂ melanic males were backcrossed to typical female offspring of the parallel line. This cycle was repeated five times to create NILs for melanic and typical forms of this moth. The effects of the melanic allele on relative fitness were evaluated in terms of developmental and reproductive characteristics. Life tables of the NILs were constructed to determine net reproductive rate (R₀). Results indicated that NILs-tt possessed apparent developmental and reproductive advantages, including shorter immature development time, higher larval and pupal survival, a shorter adult preoviposition period, and more eggs laid per female compared with the NILs-TT.