Similar in meaning:
* Surprised: This is a very common synonym for startled, often used to describe a mild reaction.
* Alarmed: This suggests a stronger reaction, often with a sense of fear or danger.
* Frightened: This implies a more intense fear than alarmed.
* Shocked: This word emphasizes the suddenness and unexpectedness of the event.
* Taken aback: This conveys a sense of being surprised and confused.
Similar in feeling, but slightly different:
* Flustered: This word suggests a state of nervousness or confusion.
* Disconcerted: This implies a feeling of being unsettled or uncomfortable.
* Nonplussed: This describes being so surprised that you don't know how to react.
The best word to use depends on the specific context and the level of surprise you want to express.