The Securities and Exchange Commission is redoubling its effort to combat the manipulation of “microcap” stocks, opening about half a dozen investigations each month into schemes suspected of bilking mom-and-pop investors, agency officials said.

As the SEC describes it, the term microcaps typically refers to the low-priced shares offered by the smallest of public companies, the ones that do not qualify to be listed on the large national exchanges, often because their stock is too thinly traded or too cheap.

Instead, microcaps are bought and sold in the less regulated “over-the-counter” market, sometimes for less than a penny per share, without much information for investors to examine. All these factors make microcaps more susceptible to fraud, which is why the SEC is intensifying its enforcement efforts in that arena.

With a new chairman at its helm, and new chiefs leading the enforcement division, the agency created a 26-person task force in July devoted to rooting out microcap trading abuses. Since then, the agency has opened five or six microcap investigations a month, well above the previous year’s pace, agency officials said. The initiative builds on the efforts of a loosely-knit SEC working group created three years ago.

“For years, we did a number of these cases, but we didn’t attack it with a systematic approach,” said Andrew Ceresney, co-director of the SEC’s enforcement division. “We now have people focused on the area full-time for the first time, and by marshalling their expertise, we think we can make a difference.”

The classic microcap scams involve efforts to hype the value of a company’s stock by spreading false or misleading information via spam, online chat groups or even paid promoters. In July, the SEC accused two such promoters, Cort Poyner and Mohammad Dolah, of paying kickbacks to individuals who agreed to sell and purchase shares in two microcap firms, thereby boosting the firms’ stock volume and prices.

This type of illegal activity, if left unchecked, can lead to “pump and dump” schemes that profit company insiders or others who take part in the fraud, and hurt unsuspecting retail investors. As soon as the price moves up, these insiders dump the shares they bought on the cheap and walk off with the difference.

Attorneys representing Dolah and Poyner declined to comment.

The fraud can take shape in many forms, including arrangements that allow the stock’s issuers to say they’re raising money for one purpose when they actually intend to use it for another. In August, for instance, the SEC accused the chief executive of a Florida software company of hiring “boiler rooms” in Europe to raise money for his firm.

Through this scheme, John G. Rizzo attracted $2.5 million from 120 investors in Britain, the SEC said. He then allegedly used the money to pay commissions to the boiler room workers, and cover his personal expenses.

Rizzo’s attorney, Steven Goldsobel, could not be reached for comment.

This week the agency continued its push when it charged Curt Kramer and his firms with buying billions of shares in two microcap firms and then failing to register them before reselling them to investors. Kramer, who did not admit wrongdoing, agreed to pay $1.4 million to cover the illicit profits he made as well as penalties.

The arrangement between Kramer and one of the companies allegedly enabled him to buy shares of that company at fire sale prices when the company was in need of money for its operations. Kramer would then unload the shares for profit, the SEC said.

The agency said that going forward it will focus on repeat offenders, who account for a substantial amount of the fraud. It also is aggressively moving to suspend trading when it has reason to suspect misconduct. The agency has issued 90 trading suspensions so far this year, up 35 percent from the same period a year earlier. Most of the suspensions involved microcaps.

But making a dent in this corner of the market will be challenging, legal experts said.

“The problem with policing fraudulent microcaps is that they are like mushrooms,” said Adam C. Pritchard, a professor at the University of Michigan Law School. “They keep popping up no matter how many you shut down.”