Legumes, in symbiotic association with Rhizobia, are able to fix atmospheric N. Six local lima bean (Phaseolus lunatus) cultivars were grown under rainfed conditions in a coastal savannah environment. Objectives of the study were to evaluate the nodulation and fixed atmospheric N levels of the six local lima bean cultivars using both the 15N isotope dilution method and N difference method (NDM). The linear relationship between fixed atmospheric N estimated using the 15N isotope dilution method and NDM, was also assessed. The experiment was arranged in a randomized complete block design (RCBD) in three replicates with seven treatments, comprising six lima bean cultivars (B1, B2, B3, B4, B5 and B6) and the early maturing local maize variety, “Doke”, as the reference crop. Total, effective nodules (EN) and non-effective nodules (NEN) were determined on 42 and 56 days after planting (DAP). The 15N isotopic dilution method and NDM were used to quantify the fixed atmospheric N by the lima bean cultivars on 60 DAP. Effective root nodules per plant (EN) on 56 DAP ranged from 0.71 to 1.22, with the lima bean cultivar B4 having the highest value and cultivars B2 and B5 having the lowest value of EN, respectively. Similarly on 56 DAP, the lima bean cultivar B4 had the highest NEN value while cultivars B1, B2 and B5 had the lowest NEN value of 0.71 per plant. The mean fixed atmospheric N was 8.98 kg·ha-1, based on the 15N isotope dilution method, which was lower than 10.13 kg·ha-1 of fixed atmospheric N determined using NDM. The linear relationship between fixed atmospheric N estimated using the 15N isotope dilution method and that estimated using the NDM, was positive but of average quality as the R2 value was 0.56. Consequently, the linear model obtained from this relationship is moderate as 56% of the data used for the linear regression analysis were accounted for by the linear regression model developed. However, NDM could be used for fast screening to select lima bean cultivars for a more detailed study to identify cultivars with promising fixed atmospheric N capabilities. Generally, results of the study provide opportunities for designing breeding and other agronomic programmes for enhancing the productivity and N-fixing capacity of local lima beans in the coastal savannah environment.