Comic books enjoy great popularity across the world and people tend to read comics on digital devices nowadays. Localizing speech balloons is a necessary step in the process of digitalizing comic books, not only because they are one of the basic elements constituting a comic page but also because they are helpful to the localization of other comic elements. However, only a few studies have been done in this direction. In this paper, we propose a clump splitting based speech balloon localization method which can detect both closed and unclosed speech balloons. The contour generated by our method is accurate at pixel level and our method works for comic books of different drawing styles. Experimental results have demonstrated that our speech balloon localization method achieves better performance than existing methods on the common evaluation measures of recall, precision and F₁ score.