NOT CONTENT to deny charges that Cuba conspired in last month's invasion of Zaire, Fidel Castro now says the charges were based on lies manufactured by presidential adviser Zbigniew Brzezinski. That is nasty - and unsupported. It is one thing to argue over the largely circumstantial - and, plainly, arguable - evidence the administration furnished to back up its assertion that Cuba 1) trained, 2) equipped and 3) "did nothing," though witting, to halt the Katangese forces. It is quite another for Mr. Castro to smear a particular official. He undercuts his denials - and his credibility - by fleeing from a discussion of the merits to ad hominem attacks.

In fact, what the quarrel with Caba most demonstrates is the inadequacy of current procedures for dealing with Havana. Committed as Cuba is to the Angolan regime, one could not really have expected Mr. Castro to tell Washington of the pending Katangese incursion - even if, as he claims, he had sought to get the Angolans to leash the Katangese. But Mr. Castro then waited almost a week after the invasion, while tensions grew, to assure Washington that the Cubans were not involved and that he had tried to halt the invasion. Privately, the administration acknowledged those assurances without - for some reason - expressing any hint of skepticism. In public, however, the administration continued charging the Cubans with preparing the Katangese and with doing nothing to head off the invasion. The resulting confusion was only compounded when the administration, in briefing congressmen, failed to convey Mr. Castro's statement that he had tried to stop the incursion. When that leaked, a challenge to the president's credibility was thrown into the brew.

Merely to reconstruct this sequence is to indicate the need for close and continuous diplomatic contact with Cuba to keep misunderstandings from aggravating real differences. But the basic problem is not poor communication. It is Cuba's policy of using its forces, in league with the arms and advisers of the Soviet Union, to resolve one after another African dispute. If Mr. Castro wishes to demonstrate good faith in Africa, let him do so in two places where conflicts are currently raging: Eritrea and Rhodesia.

In Eritrea, such a demonstration is not entirely far-fetched. Long before Cuba began helping newly revolutionary Ethiopia repel invaders from Somalia, Havana pronounced the Eritrean resistance an authentic Marxist-Leninist liberation movement. So it is that Cuba, and also the Soviet Union, are holding back from the full military commitment the Ethiopians demand to crush what they call Eritrean "secessionists." The compromise that Cuba may favor for Ethiopia and Eritrea does not seem all that different from the negotiated federal solution now endorsed by the United States. Caban and American (and Soviet) diplomats might well quietly talk about it.

In Rhodesia exists a larger opportunity for Mr. Castro to demonstrate he is no adventurist. The prospect of heavy, direct Cuban support is what has inclined the Patriotic Front to stay in the battlefield. Mr. Castro should be steering the guerrillas into political compromise. His recriminations over the last incursion are no substitute for a clear-cut commitment to fair and peaceable change in conflicts still open. At Annapolis, Mr. Carter invited "all other powers" to join in that enterprise. What is Cuba's response?