With diesel selling for as much as two dollars and ninety cents a gallon in some locations, thieves have targeted the fuel on farms.
Officials say in most of the instances someone using a siphon tube has drained the tank of a tractor sitting in a field.
But Detective Giles New of the Stanislaus County Rural Crime Unit says in some cases a thief has towed away a mobile tank filled with a thousand dollars worth of fuel.
"A couple have been fairly substantial. One tank was about 500 gallons and the second tank was probably three to four hundred gallons."
New says he fears that as diesel becomes more valuable, thieves will be more brazen and possibly dangerous.
"It may even turn into robbery-type situations instead of thefts. You know you're talking about a substantial amount of money."
Investigators say it's not difficult for thieves to find a buyer for stolen diesel fuel, since there's a robust underground economy in the Valley.