The Dow Jones industrial average dropped 136 points to 12,923 shortly after trading began. The Standard & Poor's 500 index was off 17 at 1,381, and the Nasdaq composite lost 40 points to 3,040.
The losses were broad -- only seven of stocks in the S&P 500 rose. Financial stocks fell the most. Bank of America was off 3 percent, Citigroup 2.5 percent.
The U.S. added just 120,000 jobs in March, about half the pace from December through February. The slowdown interrupted the strongest stretch of job growth since the Great Recession. The government released its jobs report on Friday, but the stock market was closed.
The stock market had already started to pull back from its strongest first quarter since 1998. The Dow closed as high as 13,264 earlier last week, then lost more than 200 points in three days.
Even before the job number came out, investors were worried that the Federal Reserve does not appear inclined to take further steps to stimulate the economy.
This week, investors will turn their attention to first-quarter corporate earnings reports. Aluminum maker Alcoa releases its results Tuesday, becoming the first company among the 30 in the Dow to do so. Two major banks, JPMorgan Chase and Well Fargo, report on Friday.
Analysts are expecting quarterly earnings to decline slightly compared with a year earlier. That would break a streak of nine straight quarters of earnings growth since 2009.
In other corporate news:
-- AOL shot up 44 percent after the company agreed to sell hundreds of patents and patent applications to Microsoft for a little more than $1 billion. The company plans to return some of the cash to shareholders.
-- Avon fell 3 percent after the struggling beauty products company named a former executive at Johnson & Johnson, Sherilyn S. McCoy, to be its CEO. She starts April 23.