This paper addresses gender differences in international research collaboration measured through international coauthorship. The study is based on a dataset consisting of 5,554 Norwegian researchers and their publication output during a three-year period (43,641 publications). Two different indicators are calculated. First, the share of researchers that have been involved in international collaboration measured by co-authorship, and second, the share of their publications with international co-authorship. We then develop an index which takes both these indicators into account: The Gender Difference Collaboration Index. The study shows that there are distinct gender differences in international research collaboration in Norway at an overall level. However, when the data is analyzed by scientific field, academic position and publication productivity of the researchers, the gender differences are less pronounced and in some cases, women have higher collaboration rates than men. The differences are largest for personnel in recruitment positions and for less productive researchers.