The current research examined performance of good and poor readers of Thai on two tasks that assess sensitivity to dynamic visual displays. Readers of Thai, a complex alphabetic script that nonetheless has a regular orthography, were chosen in order to contrast patterns of performance with readers of Korean Hangul (a similarly regular language but one that has a simple visual format, see Kim & Davis, 2004). Thai poor readers were less sensitive than good readers on the two measures of dynamic visual processing; they had higher thresholds for detecting coherent movement and required longer ISIs to report group movement in the Ternus task. These results differ from those for poor readers of Korean Hangul that found no relationship between visual processing thresholds and reading skill. This contrast suggests that the expression of visual processing problems in dyslexia is mediated by the format properties of the writing system and points to the need to consider such factors in formulating brain-behavior relationships.