The purpose of this study was to investigate bilateral deficit (BLD) of force in submaximal isometric muscle contraction and effect of muscle fatigue on BLD. In Experiment 1, nine male students were subjected to investigate whether BLD occurs in submaximal force generation. They were asked to generate maximal force of the elbow flexors with their right, left, and right and left arms simultaneously. They also performed submaximal force generation at 30% and 60% of MVC in a way similar to that for the maximal force generation. The magnitude of BLD was compared between maximal and submaximal conditions by Bilateral Index: BI=100× (Bilateral Right and Left force/Unilateral Right force + Unilateral Left force) - 100. The mean values of BI during maximal. 30%, and 60% force generations were -9.4, -10.6, and -7.4, respectively, and there was no signi-ficant difference between conditions. This suggests that BLD occurs similarly regardless of the level of force output. Effects of the other arm starting to generate force (30%MVC, 60%MVC, MVC) on force level of one arm already holding a target force (30%MVC, 60%MVC, MVC) for 2 s were also investigated additionally. The magnitude of BLD was significantly larger for the additional condition compared to that seen in the simultaneous force generation by both arms from the beginning. To examine BLD during repeated muscle contractions, 14 male students performed 100 maximal isometric elbow flexions with right and left arms simultaneously (2-s contraction and 4-s rest) with right and left arms alternatively (2-s contraction and 4-s rest for each arm) in Experiment 2. BI values were calculated over the 100 contractions. During 100 maximal isometric contractions, force decreased to approximately 70% of the initial force at the end. The BI value increased from -8.6 (1st) to -0.1 (25th) then 3.3 (100th) during exercise, indicating that BLD disappeared as the muscular fatigue proceeds.