Tell them. Knowing you had an agent before can give weight to your query—it means you’ve caught someone’s eye once before. My agent, Erin Murphy of the Erin Murphy Literary Agency, says that while agent reactions can vary, she isn’t put off to learn someone had a previous agent. But she doesn’t dig the term “creative differences.” Erin says, “Much as I know agents and writers part ways for many reasons having nothing to do with either of them being troublemakers, of course there is always a tiny voice wondering why things didn’t work out with that agent, and that phrase sounds more of an alarm than many explanations; I immediately begin to suspect that this writer doesn’t like to listen to guidance. Something like ‘differing opinions about which direction to take my writing next’ sounds less ominous to my ear—it essentially means the same thing but can be taken as ‘that agent wanted me to write to a particular market and my muse wouldn’t cooperate,’ which I’m sympathetic to.” In the initial query, though, something like this will do you just fine: “In November 2009 I amicably parted ways with my agent of three years and I am now seeking new representation.” There are other factors, of course. Did the other agent sell anything? How widely circulated were the manuscripts? Is there any new material that the agent had not sent out, which would give a new agent space to build from? But you can get into that later, after this new agent has nibbled.