The decline in openings comes after a sharp rise the previous two months, driven by temporary government hiring for the 2010 census and more openings in the private sector.
The Labor Department said Tuesday that job openings fell to 3.2 million in May from 3.3 million in the previous month. April's upwardly revised figure was the highest in 18 months.
The department's report, known as the Job Openings and Labor Turnover survey, illustrates how competitive the job market is. There were about 4.7 unemployed people, on average, for each job opening in May. That's down from the peak of 6.3 last November, but is much higher than the 1.8 unemployed per opening when the recession began in December 2007.
Job openings have rebounded from the depths of the recession. May's total is 37 percent above the low point of 2.3 million openings in July 2009. But it's still far below pre-recession levels of about 4.5 million.
Layoffs increased by about 100,000 to 1.9 million in May, the department said, but remain at pre-recession levels. The department said layoffs rose to a peak of 2.6 million in January 2009.
The government's job openings report echoes other recent data that shows hiring by private employers weakened in May and June, heightening concerns that the economic recovery is slowing.
Businesses added a net total of only 83,000 jobs in June and 33,000 in May, after net gains of 200,000 in March and April. The economy needs to generate at least 100,000 new jobs per month just to keep up with the rising population, and twice that level to rapidly reduce the unemployment rate, currently at 9.5 percent.