This paper concerns a discrete-time Geo/Geo/1 retrial queue with both positive and negative customers where the server is subject to breakdowns and repairs due to negative arrivals.The arrival of a negative customer causes one positive customer to be killed if any is present,and simultaneously breaks the server down.The server is sent to repair immediately and after repair it is as good as new.The negative customer also causes the server breakdown if the server is found idle,but has no effect on the system if the server is under repair.We analyze the Markov chain underlying the queueing system and obtain its ergodicity condition.The generating function of the number of customers in the orbit and in the system are also obtained, along with the marginal distributions of the orbit size when the server is idle,busy or down.Finally,we present some numerical examples to illustrate the influence of the parameters on several performance characteristics of the system. This paper concerns a discrete-time Geo / Geo / 1 retrial queue with both positive and negative customers where the server is subject to breakdowns and repairs due to negative arrivals. The arrival of a negative customer causes one positive customer to be killed if any is present, and simultaneously breaks the server down.The server is sent to repair immediately and after repair it is as good as new.The negative customer also causes the server breakdown if the server is found idle, but has no effect on the system if the server is under repair. We analyze the Markov chain underlying the queuing system and obtain its ergodicity condition. The generating function of the number of customers in the orbit and in the system are also obtained, along with the marginal distributions of the orbit size when the the server is idle, busy or down. Finally, we present some numerical examples to illustrate the influence of the parameters on several performance characteristics of the system.