Yorkshire Water sparked an anthrax scare after sending out a mailshot containing white crystals.
There were a number of anthrax attacks in the US last year
West Yorkshire Police received a number of emergency calls from people in the Leeds area who received white A4 envelopes bearing no company logo.
One resident put on rubber gloves and opened the package in his garden when he heard a rustling in the envelope.
The envelopes contained a "Save-a-Flush" device filled with silica sand designed to expand when placed in a toilet cistern.
This reduces the amount of water used when the toilet is flushed and in turn water consumption.
The mailing was part of Yorkshire Water's ongoing campaign to encourage people to conserve water in the home.
But a West Yorkshire Police spokeswoman said: "We received a number of phone calls from alarmed customers who feared the contents were more sinister.
"In view of last year's anthrax attacks in America, the recipients feared a repeat phenomenon."
A spokesman for Yorkshire Water said: "The decision to send out these water conservation devices in plain envelopes without the Yorkshire Water logo on was
"Although there was no Yorkshire Water logo on the envelopes, each one did, however, contain an explanatory letter printed on company stationery and a water
conservation leaflet which also bears the Yorkshire Water brand."