I've seen this claim stated in many places, for example the talk.origins FAQ:
Of the scientists and engineers in the United States, only about 5% are creationists, according to a 1991 Gallup poll (Robinson 1995, Witham 1997). However, this number includes those working in fields not related to life origins (such as computer scientists, mechanical engineers, etc.). Taking into account only those working in the relevant fields of earth and life sciences, there are about 480,000 scientists, but only about 700 believe in "creation-science" or consider it a valid theory (Robinson 1995). This means that less than 0.15 percent of relevant scientists believe in creationism. And that is just in the United States, which has more creationists than any other industrialized country. In other countries, the number of relevant scientists who accept creationism drops to less than one tenth of 1 percent.
Original source: Newsweek?
Now, with some source-hopping I found the source of the claim to be a magazine article:
Larry Martz & Ann McDaniel, "Keeping God Out of Class (Washington and bureau reports)". Newsweek (Newsweek Inc.) 1987-JUN-29, Pages 22 & 23. ISSN 0028-9604.
I don't know if this is the original source, or if it refers to a scientific study. I'd like to know that.
Quote from the Newsweek article
By one count there are some 700 scientists with respectable academic credentials (out of a total of 480,000 U.S. earth and life scientists) who give credence to creation-science, the general theory that complex life forms did not evolve but appeared 'abruptly'.
Now, I can't get my hands on the Newsweek article in question. However, it sounds to me that there's one magazine article from 1987 that talks about "one count" without references. And this gets cited all around the web.
Does the Newsweek article say any more about this count? Is there some proper research behind the claim, and if so, what are the methods for counting? Is the fraction of creationists in life scientists in the US really only 0.15%?
I say only because using a source talk.origins used, too, in 1997 a full 44% of American adults believed in creationism. Of all scientists, the amount was 5%. I'm used to seeing figures like these, so 0.15% for a specific group of scientists seems very little. However, if only is offensive to some and removed again, I'm not going to argue.