Yes, a defendant can be convicted solely based on the testimony of a co-conspirator. However, the prosecution must provide credible and reliable evidence that supports the co-conspirator's testimony. The co-conspirator's testimony must also be corroborated by other evidence presented in the case. The defendant may try to discredit the co-conspirator's testimony by arguing that it was given in exchange for leniency or immunity. Ultimately, it is up to the judge or jury to decide if the co-conspirator's testimony is sufficient to prove guilt beyond a reasonable doubt.