In order to protect the value of your trademark, you should hire a lawyer and send a letter demanding that the other company stop using the similar mark because it is confusing. There are many factors consider when evaluating the potential for confusion. For example, if the other company started using their mark first (before your use started), regardless of registration, that company might have some common-law trademark rights in the name.
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You have an options that you can take which include sending a cease-and-desist letter to the other company and filing a lawsuit against the other company for trademark infringement.
What you should do is meet with a trademark attorney and discuss your situation in private with the attorney. The attorney will be able to give you an idea as to whether the courts would find a likelihood of confusion between the two marks. In addition, the attorney will be able to do the research to determine who used its mark first and therefore has priority to use.
Registering a trademark with the USPTO (which is what I am assuming you mean by 'trademarked') only prevents others from adopting marks within the U.S that present a likelihood of confusion after registration of the mark. Companies already using a mark that presents a likelihood of confusion may continue to use their marks in the geographical areas where the companies were using the mark prior to registration.
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With your other posting, five very good answers.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Before jumping to a cease and desist letter, you may want to look into whether the other company has prior rights in the brand. An attorney can help you with this analysis.
I agree with attorney Weiner. If the other company was using the mark prior to your own use of the mark (or before you filed a Sec. 1(b), intent-to-use application for the mark), they, not
you, may have superior rights, even if you have registered the mark and they haven't.
There is only one action you can prudently take. See a trademark litigation attorney immediately. Do not try to play amateur lawyer or you may be headed for an expensive lesson in trademark litigation strategy.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.