Abstract: Scientific journal publishers have rapidly converted during the past 25 years to predominantly electronic dissemination, but the reader-pays business model continues to dominate the market. Open Access (OA) publishing, where the articles are freely readable on the net, has slowly increased its market share to near 20 percent but has failed to fulfill the visions of rapid proliferation predicted by many early proponents. The growth of OA has also been very uneven across fields of science. We report market shares of open access in 18 Scopus-indexed disciplines ranging from 27 percent (agriculture) to 7 percent (business). The differences become far more pronounced for journals published in the four countries that dominate commercial scholarly publishing (US, UK, Germany, and the Netherlands). We present contrasting developments within six academic disciplines. Availability of funding to pay publication charges, pressure from research funding agencies, and the diversity of discipline-specific research communication cultures arise as potential explanations for the observed differences.