US$ Dollars is the currency used in Ecuador and the Galapagos.
There are NO banks on Isabela. It is important to bring sufficient cash for the duration of your stay on the island. Bring small bills – many places will not (be able to) accept or change 50 or 100 dollar bills.
On the neighboring island of Santa Cruz, the Banco del Pacífico is open Monday to Friday 8:00 - 3:30; Saturday 9:30 -12:30. The bank cashes travelers checks; however personal checks are not accepted. The bank's ATM accepts only Cirrus, Star, and MasterCard (7 am - 11:30 pm). Unfortunately, the ATM is down more often than the bank would like to admit. If this is the case you can request a cash withdrawal with your MasterCard (but not with a Cirrus card) from a bank teller. Money transfers are possible; however they take 4 to 7 days. Faster but more expensive is a wire transfer with Western Union or MoneyGram both of which are available in Isabela. With a MoneyGram you will receive your dinero within minutes -- if the phone lines are working. You do not have to be a card holder to use a MoneyGram.
Credit cards are only accepted in the hotel Casa de Marita or at the local airline in Isabela. On the island of Santa Cruz Mastercard, Visa, and American Express are usually accepted (only in higher-end places). In general, Mastercard is the most widely accepted card both in the Galapagos and on the mainland.