The effects of 5 wk of equivalent intensity, 85-90% maximum heart rate (MHR), run-only (N = 6) vs cycle/run (N = 5) training were evaluated in moderately fit runners, mean VO2max of 55.2 ml.kg-1.min-1, 19-35 yr old, on maximal treadmill (TM) and cycle ergometer (CE) testing, 5000-m and 1609-m run performances, and submaximal measures while treadmill running. Subjects participated in either 4 d.wk-1 of run-only or alternating run and cycle training. Both groups significantly improved TM VO2max (P < 0.05), CE VO2peak (P < 0.01), and 5000-m (P < 0.01) and 1609-m (P < 0.05) run times with no significant differences between the groups. The TM VO2max pre/post values were 55.3 +/- 3.0 to 58.2 +/- 3.0 and 55.6 +/- 3.8 to 58.9 +/- 2.4 ml.kg-1.min-1 for the run group and cross-trained group, respectively. Posttraining submaximal treadmill running showed statistically significant pace increases of 18.7 (run) and 16.1 (cycle/run) m.min-1 with similar heart rate, blood lactate, and RQ values as pretraining. Results indicate that 5 wk of either mode of training can significantly improve aerobic capacity and run performance.