Ammo was founded seven years ago and labels itself as creating "irreverent" and "word-of-mouth" campaigns for clients including Cadbury Schweppes, Remy Cointreau and Miller Brewing.
As part of the deal, Ammo partner Julian Aldridge will become president of the agency as well as another Isobar outfit, Freestyle Interactive, whose clients include Electronic Arts and Dolby.
Ammo's other partners Kerry Lange and Andrew Strickman will continue with their management roles at the company.
Aldridge said: "What Isobar gives us is the opportunity to continue to expand our service offering to our clients both in the US and internationally."
David Verklin, Aegis Media America and Asia chief executive, said: "Word-of-mouth marketing truly will be the next big battleground for marketers who understand that with key peer influences you can reach hundred of people in a very significant way."
Also part of Isobar's US network are Carat Fusion and last year's purchases, technology consulting company Molecular and internet search-marketing agency iProspect.com.
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .