Yellow Book has been gradually closing the gap with its U.S. IYP rivals, which testifies to the publishers’ increasing emphasis on its online business as a driver of future growth. According to data recently put out by comScore, Yellowbook.com was the fastest growing U.S. Internet site in the month of March.

I spoke earlier today with Yellow Book’s chief new media officer, Pat Marshall, who cited SEO efforts and promotion as key elements of the growth. He also conceded that while Yellow Book has made tremendous strides, the publisher still has a ways to go to catch up to the market leaders. Marshall will be a featured speaker at the upcoming “Drilling Down on Local” event in Seattle.

A report issued by comScore last month showed that the “Yellow Book Network” has increased its share of IYP searches from 4 percent in Q4 2006 to an 8.6 percent share in Q4 2007. Growth in unique visitors for this same period was 137 percent, from 4.4 million to 10.4 million.

R.H. Donnelley also made great progress in the same time frame, aided by its Business.com acquisition, growing 123 percent to 6.8 million uniques.