David Nchoko, a clerk in the Masai Mara game reserve, and the former head warden, Simon ole Makallah, were accused of killing Ms Ward, 28, in September 1988. Their trial was due to start in Nairobi next week.
Ms Ward's mother, Jan, from Bury St Edmonds, Suffolk, said yesterday: "We heard from Kenya earlier today that the case against David Nchoko has been thrown out through insufficient evidence."
John Ward has spent the past 10 years trying to identify his daughter's killers and bring them to justice.
It is believed the case will proceed against Mr Makallah.
A Foreign Office spokesman said: "David Kandula ole Nchoko was today acquitted of the murder of Julie Ward because of the lack of evidence. Simon Makallah's next hearing will be on 26 February."