Search is a ubiquitous part of the online user experience, so it's only natural that major corporations want to appear as high as possible in search results. Yet a surprisingly large number of America's top companies are failing to do so, according to a study by SEO firm Conductor, Inc.

Conductor found that almost two-thirds of the Fortune 500 companies have little or no visibility in the search results for their most advertised keywords. The study examined 2,197 domain names and 2.4 million keywords and graded the Fortune 500 companies on natural search visibility and optimization effectiveness.

The majority of companies were doing an "extraordinarily poor job" with SEO, according to Conductor, with only 28 percent receiving a passing grade, meaning they showed "legitimate presence" in search results. Several companies had absolutely no natural search visibility.

A measly 8 percent of the Fortune 500 showed strong SEO presence, while less than 3 percent of the Fortune 500-owned domains appeared in the top results of searches for relevant keywords. More than 80 percent of all activity in search comes from natural results, according to Conductor.