Abstract: Background: Transversus Abdominis (TrA) has an important role in spinal stability contributing to both intra-abdominal pressure and the formation of a muscular corset around the spine. Transversus Abdominis has been shown to be dysfunctional in patients with low back pain (LBP) and ultrasound (US) imaging is seen to have an increasing role in the assessment and management of these patients. This study assessed the intra-rater reliability of a novice operator to measure the thickness of transversus abdominis when using brightness (B) mode ultrasound imaging. Method: Twenty normal subjects participated in an operator blinded study to assess the intra-rater reliability of US in measuring the thickness of TrA when contracted using the abdominal hollowing exercise (AHE). Brightness (B) mode US was used to image the TrA at the end of expiration. Subjects were then re-imaged in the standardized position. Results: An intraclass correlation coefficient of 0.96 with 95% confidence intervals of 0.93- 0.98 were obtained indicating excellent reliability. However, limits of agreement were −1.52 mm to +1.35mm (range 2.87mm) suggesting that clinically meaningful measurements may not reliably be measured by a novice operator. Conclusion: This study indicates that additional training is required to achieve clinically meaningful results. It also highlights the importance of using more than one method of establishing reliability.