Speculators in IMM yen futures added to a net short position in the week ended September 28, data from the Commodity Futures Trading Commission showed on Friday.
"Short" positions are effectively bets that a specific currency will fall.
The data from the CFTC's Commitments of Traders report on speculative positioning are used by analysts as an indicator of future market direction.
For example, extreme net long speculative positions often signal a decline in the currency going forward, especially if that position conflicts with the positioning of the more influential commercial players.
Speculators generally are trend followers seeking to pick a precise top or bottom in the market.
Yen futures traders grew the net short position to 22,203 contracts, its largest since March, from 9,357 contracts reported in the previous week.
During the CFTC's reporting period the dollar rose vs the yen to a close of 111.35 yen on September 28 from a close of 110.55 on September 22.
Meanwhile, speculators in IMM Canadian dollar futures grew a net long position to 46,756 contracts, its largest on record, from 39,399 reported a week earlier.
During the CFTC's reporting period, the US dollar fell against the Canadian dollar to a close of C$1.2718 on September 28 from a close of C$1.2826 on September 22.