Bad sign? Retail investors all in: TD Ameritrade

Retail investor 'fully invested': TD Ameritrade CEO

A broad look at the 6.5 million customer accounts at TD Ameritrade indicates that retail investors are "pretty fully invested" in stocks, the online brokerage's CEO said Thursday.

Fred Tomczyk cited several signs of this: margin loans at high levels, client cash at low levels and account holders at the firm logging in frequently. "It's usually a good indication that people are very engaged in the markets and watching their investments closely," he said on CNBC's "Squawk Box."

But Tomczyk acknowledged the potential pitfalls of these trends and what they may portend for stocks. "I wouldn't be surprised if we have a correction here. We've had six [or] 6½ years of up markets here."

The Dow Jones industrial average and the were relatively flat on the year as of Wednesday's closing. "I think last year it wasn't an overly strong market. This year it hasn't been an overly strong market," Tomczyk said. "I think it's all going to depend on what the data tells us about the U.S. economy in the next three to six months."

Many economists are expecting a pickup in growth in the second half of the year, after the government on Wednesday reported that first-quarter gross domestic product grew at a lower-than-expected 0.2 percent. The economy advanced 2.2 percent in the fourth quarter and 5 percent in the third quarter of 2014.

In a policy statement Wednesday, the Fed played down the soft economic growth and removed all calendar references to when an interest rate hike may be coming.

Liftoff in June seems to be pretty much out, with September seen as the most likely month for the first rate increase in nearly a decade. But Tomczyk believes the Fed won't move until later. "What the forward [yield] curve would tell you right now [is] short term rates aren't going to move until December."

Earlier on "Squawk Box," UBS economist Drew Matus argued for a June move. "They had June lined up in their sights as of March."

"I think everything would be functioning a lot better now," if the Fed didn't back off, he said.

Read MoreWhy Fed should hike in June: UBS economist

Top holdings Apple, Exxon Mobil

Looking at individual stock names, Tomczyk said, "Apple is our most widely held stock. [It] has been for a long time."

He also said clients are heavily invested Exxon Mobil, which has fallen about 5 percent in 2015 along with the 40 percent decline in oil prices over the past 12 months, though U.S. crude has been recovering somewhat.

Read MoreExxon earnings, revenue beat by a mile

"We have [a] large active trader base and so they tend to have a contrarian view," said Tomczyk, whose company is celebrating its 40th anniversary this year.

Last week, TD Ameritrade reported earnings of 35 cents per share for its fiscal second quarter, which were were in line with expectations. Revenue of $803 million was below forecasts for $821 million.

Earlier this month, rival Charles Schwab missed on the top and bottom lines, with earnings of 22 cents per share on revenue of $1.53 billion in the first quarter.