Context: Deep water running (DWR) is an aquatic aerobic exercise which involves running in water without the feet touching the bottom of the pool, and it may involve different activation of trunk muscles compared with running or walking on land. This form of exercise is gradually being adopted as a form of therapeutic exercise for people with low back pain. It is proposed that different types of running or walking in water may be a more comfortable form of training for the trunk and abdominal muscles compared with exercising on dry land. Objectives: This study aimed to examine the trunk muscle activation in DWR in 2 different styles—high knee style and cross-country style, and these were compared with walking on land. Participants: Eleven healthy individuals (2 females and 9 males, mean age = 24 [4.6]) were recruited for this study. Outcome Measures: Surface electromyography was used to examine the activities of the right transversus abdominis, rectus abdominis, lumbar multifidus, and lumbar erector spinae muscles in 5 conditions: static standing on land and in water, running in deep water with high knee and cross-country styles, and finally walking on a treadmill. Results: The percentage of maximal voluntary contraction of the transversus abdominis was significantly higher for both running styles in DWR, compared with that of static standing in water. Comparing directly the 2 styles, muscle activity was higher with a high knee action compared to without. The activation of transversus abdominis during high-knee DWR was comparable with that during treadmill walking and this may have clinical implications. Conclusion: The results of this study confirmed that running in deep water with a high knee action activated trunk muscles differently compared with standing or walking on land.