Fidelity, which claims to be the nation’s leading provider of workplace retirement savings plans, says the number of its plans offering auto enrollment rose almost 70% from year-end 2007 through March 31, 2009, to nearly 2,700 defined contribution plans. Still, that’s just 16.3% of 401(k) plans that Fidelity manages.

Among auto enrollment participants, 52% were between the ages of 20 and 34. Among those between the ages of 50 and 64, that number dropped to 13%. The reason, says Fidelity, is that older participants are either already participating in a plan or elect to enroll on their own.

And the majority (56%) of participants automatically enrolled made less than $40,000 a year. Only 10% among the auto enrollment group had higher salaries ranging from $80,000 to $150,000 a year.

"Auto enrollment is doing exactly what it was intended to do," said Scott B. David, Fidelity Investments’ president of workplace investing. "It is driving many Americans who would have otherwise not saved—mostly the young and lower-paid employees—to begin saving early and consistently which is critical to having a healthy income in retirement."

And once they’ve participated in the auto enrollment option, only 10% of employees opt out of the plan, Fidelity says.