They were among nine Canadians and Americans - two of them expats in Thailand - indicted on charges of bilking investors in about 35 countries of more than $140 million, the US Attorney's Office for the Eastern District of New York said.
A seventh suspect was arrested in the Canadian province of Ontario, while two others remain at large.
Those arrested were named as Kolt Curry, 38 and Gregory Ellis, 46, both Canadians from Ontario; and Americans Gary Kershner, 72, of Arizona, Joseph Manfredonia, 45, of New Jersey, Cort Poyner, 44, of Florida, and Sonkram Roy Sahachaisere, 43, and William Seals, 51, both of California.
Still at large were Sandy Winick, 55, and Greg Curry, 63, both Canadians whose place of residence was given as Bangkok.
"We are currently working with our international partners to seek the extradition of Winick and Curry,'' US Attorney's Office spokeswoman Zugiel Soto told AFP.
"The indictment and arrests are the result of one of the largest international penny stock investigations ever conducted by the Department of Justice and the Federal Bureau of Investigation (FBI),'' with help from foreign police, the US Attorney's Office said.
According to the indictment against them, the defendants fraudulently "pumped up'' the share price of worthless penny stocks and then "dumped'' billions of shares of those stocks by unloading them on unsuspecting investors worldwide.
They also allegedly operated "boiler room'' call centres in at least four countries that induced penny-stock investors to pay advance fees that the defendants promised would enable them to sell their stocks and recover losses. In fact, the fees were stolen.
"I tell you what man ... hitting the Americans would be like taking money from a baby,'' one of the defendants was heard saying in an FBI wiretap.
If convicted, the nine face up to 20 years in prison for each count of conspiracy, wire fraud and securities fraud, as well as time behind bars for impersonating an agent of the Internal Revenue Service (IRS), the US tax collection agency.
Those arrested in the United States appeared in federal courts in New York's Brooklyn borough as well as in Miami, Los Angeles and Tucson, Arizona.