Michael J. Zimmer, a law professor at Loyola University Chicago, said a broad perceptual shift was needed. “The first level is to get the attention of the industry, that this is a real problem,” he said. “It’s sort of assumed that it’s a given, a state of nature, and of course it’s not.”

The A.C.L.U. did not name specific offenders, but requested that the agencies use studies and hiring information from major studios, networks and the Directors Guild of America to identify employers with the worst records.

The letters cited numerous examples of bias: for instance, a University of Southern California study found that of the top-grossing 100 films from 2013 and 2014, just 1.9 percent were directed by women. A Directors Guild of America analysis of 220 television shows consisting of 3,500 episodes broadcast in 2013 and 2014 found that 14 percent were directed by women. A third of all shows had no female director at all.

Image Actress Amy Schumer accepts the CinemaCon Breakthrough Performer of the Year Award during The CinemaCon Big Screen Achievement Awards at Caesars Palace in Las Vegas. Credit... Steve Marcus/Reuters

Propelling this paucity, the A.C.L.U. said, is an industrywide boys’ network in which gender discrimination is often on overt display. The organization collected stories from 50 female directors who reported being told by executives that a show was not “woman friendly”; learning that producers had repeatedly told agents to “not send women” for prospective jobs; or being informed at a meeting for a television job that “we already hired a woman this season.”

One female director with decades of experience said Hollywood worked by its own rules, which can be alluring but also allowed executives to say wildly inappropriate things. “Sometimes showrunners will say, ‘This isn’t a good show for a woman director,’ or, ‘Our actors are hard on women,’ ” said the director, who spoke on condition of anonymity because she feared career repercussions. “Or they’re approaching it as if ‘We’re protecting you by not giving you this job.’ That way they turn it on its side, to make everything O.K.”