Could depend on your jurisdiction and/or the type of activity, but in most cases (in my unqualified opinion) the answer is Yes. Doesn't really matter if it's bitcoins, foreign currency, or even other goods/services (received as a payment), you're still expected to pay tax on it (providing it's a taxable income), at the exchange rate/value at the time of receiving payment.

Let us know what kind of earnings are you talking about and what country do you live in and maybe someone will offer you a sound advice.