This is an interesting and a complex question. I will answer the general principles that applies, but in keeping with the Kollel rules I am unable to answer Halach L'Maaseh.

The first question is, when one buys stocks, does one become an actual partner in the business? If this were so, any company that has had its stocks purchased by a Jew (i.e., essentially all of them!) would be problematic. Although according to the strict interpretation of what stocks are (partial ownership in a business), L'Halacha one is not considered a "partner" until one owns a "controlling interest". This definition is used Halacha l'Maaseh by various Vaadei Kashrus when determining if a company has a problem of "Chometz She'Ovar Olov HaPesach" due to Jewish "ownership" (See: Igros Moshe Even HaEzer Vol. 1 Siman 7. See also Melamed L'Ho'il OC 91 and a short Teshuva from Rav Shlomo Kluger Siman 238 (Marei Mekomos courtesy of HaRav Tzvi Hollander Shlita of Star K Vaad HaKashrus). L'Cho'oira, if one is trading options or ETFs the whole question is irrelevant. When buying mutual funds, the nature of the owner of the funds would also be relevant although this is usually a publicly owned company. The Igros Moshe states that one may receive interest from a corporation even if run by Jews as a corporation itself is not a Jew and you are not receiving interest from a Jew; according to this opinion, one may purchase corporate bonds (which are essentially a loan to the company) from any corporation. However, if I remember correctly there are other opinions that forbid this, and please consult a Halachic authority before taking action.

All of the is regarding the actual stocks themselves. However, since one must trade through a brokerage, the picture is different. If the brokerage is a publicly owned company, then it is logical to assume that the above parameters apply there as well. But if, on the other hand, the brokerage is a private company with Jewish ownership, or a small partnership or even an individual running a brokerage office, the situation might very well be prohibited without a "Heter Iska" as you are depositing money with the individual and receiving more. You are not buying the stocks directly; you are "loaning" him money and he is buying stocks for your account which he manages. However if one trades through one of the major commercial brokerages, this should not be a problem.

As an interesting note: The brokerage Interactive Brokers (IBKR) is a publicly traded company which is controlled by Tomas Peterffy who controls about 75% of the company. In addition, a significant portion of the company is controlled by employees. If we assume that some of them are Jewish, and according to media reports Peterffy is also Jewish we seem to have a conundrum on our hands. However, it seems that the truth is that Peterffy is not Jewish and I have consulted with a Halachic authority who reported that it appears that IBKR may be used like other publicly owned brokerages.

I hope this gives a good start to understanding the issues, and once again, for Halacha L'Maaseh please consult your own Rav.

Shimon Brodie