"Doesn't this Cuban espionage comic strip seem pretty ridiculous?" Castro asked in an article on Saturday.
Castro, however, did not refute accusations that a retired State Department official and his wife had spied for Cuba for almost 30 years.
His article came after the US government on Friday accused Walter Kendall Myers and his wife Gwendolyn Myers of conspiracy to hand over classified information to Havana.
He also questioned that the timing of their arrests was politically motivated.
The story emerged just 24 hours after "the defeat suffered" by US diplomacy when the Organization of American States cleared the way for Cuba to rejoin the body, Castro wrote.
Myers and his wife were arrested on Thursday in Washington after a three-year investigation that began before Myers' retirement from the State Department in 2007.
Myers had been under suspicion since 1995 and FBI investigation since 2006.
If the couple had been watched that long, "why were they not arrested before?" Castro asked.
