Bundeskartellamt, the German competition authority, imposed fines totalling €13.2m on Hans-Willi Böhmer Verpackung und Vertrieb GmbH & Co. KG (Böhmer) and Kartoffel-Kuhn GmbH (Kuhn).
Böhmer is one of the two main potato and onion packaging plants while Kuhn is only in south-west Germany. Böhmer and Kuhn are the major suppliers of packed potatoes and onions to the Metro group.
Since early 2005 and until proceedings started in May 2013, the responsible people at the firms had been in regular telephone contact, especially in the run-up to the weekly offer for packed potatoes and onions to the Metro group.
Action started with a sector-wide dawn raid following a leniency application.
Fines are not yet final and can be appealed to the Düsseldorf Higher Regional Court.
Andreas Mundt, president of the Bundeskartellamt, said: "For years the companies concerned have agreed on an important parameter in the calculation of their weekly offers to Metro.
"By aligning their purchase prices used in the calculation, the two major suppliers of the Metro group virtually eliminated any price competition between them."