Read the whole thing, as they say, but here's a key paragraph:
Since I began to publish in 1962, I have often been asked, by people of all colors, what my experience of racial prejudice in the science fiction field has been. Has it been nonexistent? By no means: It was definitely there. A child of the political protests of the ’50s and ’60s, I’ve frequently said to people who asked that question: As long as there are only one, two, or a handful of us, however, I presume in a field such as science fiction, where many of its writers come out of the liberal-Jewish tradition, prejudice will most likely remain a slight force—until, say, black writers start to number thirteen, fifteen, twenty percent of the total. At that point, where the competition might be perceived as having some economic heft, chances are we will have as much racism and prejudice here as in any other field.