Microsoft's web browser dropped 0.7 percentage points to finish the month at 59.95 percent. This is the first time that IE has fallen under the 60 percent mark.
IE's decline was less than the previous month - it lost nearly a full point of share in March - but was still above its average monthly decline.
Google's Chrome was the only browser to gain significant usage share last month, and again trounced rival open-source Firefox.
By the end of April, Chrome accounted for 6.7 percent of browsers.
Chrome boosted its share by 0.6 percentage points, by far the largest increase of any browser for the month, and the second-highest increase since Google launched the program in September 2008.
Mozilla's Firefox was up last month, albeit by only 0.07 percentage points to 24.6 percent.
April was the second month in a row that Firefox trended up, a victory of sorts after losing ground for four straight months starting in November 2009.
Firefox has yet to reach the 25 percent milestone.
Last month, Vince Vizzaccaro, executive vice president of Net Applications, noted that Firefox was "just holding steady", and gains that had once come easily to it were instead heading Chrome's way. That appeared to be the case again in April.
Mozilla's bright spot was that it has convinced nearly two-thirds of its users to upgrade to the newest Firefox 3.6, which launched in January.
By the end of April, 62.3 percent of all Firefox users were running the newest edition, while 23.6 percent ran 2009's Firefox 3.5 and 11.2 percent ran the now-unsupported Firefox 3.0.
Microsoft's newest browser also showed strong gains last month as it grew by one percentage point to 24.7 percent; when its 'compatibility view' is included, IE8 accounted for 27.6 percent of all browsers.
The older IE6 and IE7 both lost share in April, ending at 17.6 percent and 12.5 percent, respectively. IE7's losses are slowing, however; its three-month average decline is less than half that of the 12-month average.
IE6, which celebrates its nine-year anniversary this August, continues to lose share, but by Net Applications' numbers, it will be a force for some time.
If IE6 stays on its downward pace of the last three months, the aged browser won't slip under 10 percent until the summer of 2012.
Net Applications measures browser usage share by collecting systems data from the computers that visit the 40,000 sites it tracks for customers of its analytics services.
See also: 10 Firefox add-ons for business