All churches in a French diocese have been ordered to keep consecrated hosts in secure places, rather than tabernacles in some cases, after thefts.
Bishop Pascal Roland of Bellay-Ars diocese told priests to leave tabernacles open to show they contained no valuable ciboria.
But hosts could be left in a tabernacle if it is made of metal and securely locked, or if there are enough believers present to deter thieves, the bishop said.
His order can after five such thefts in the first week of February, the latest in a series of incidents in local churches.
In most cases, the consecrated hosts were simply dumped in the church, but some have also disappeared with the ciboria.
The thefts are the latest in a growing trend of acts of vandalism or discrimination against Christians throughout Europe.