University students' achievements in organic chemistry depend on cognitive variables. In addition, non-cognitive variables such as anxiety levels also have an impact on students' organic chemistry achievements. The aim of this study was to develop a measurement tool assessing the anxiety levels of university students in organic chemistry lessons. In this study, the Organic Chemistry Anxiety Scale (O-CAS) consisting of 24 items was developed, its validity and reliability was analysed. All the items are positively worded to indicate increased anxiety. Factor analytic evidence from a sample (n=340) of university organic chemistry students indicated that the O-CAS measured three constructs. Additional analysis with a second sample (n=297) showed that scores on these anxiety constructs were internally consistent, with Cronbach's alphas ranging from 0.87 to 0.92 and were 0.95 for the overall scale. Further, the result of analysis of the third sample (n=195) indicated that there was a statistically significant relationship between organic chemistry anxiety and organic chemistry achievement of students. According to these results, the O-CAS can be used as a valid and reliable instrument in chemistry education.