Hydrogels　of　numerous　chemical　compositions　have　achieved　high　fracture　toughness　on　the　basis　of　one　physical　principle.　As　a　crack　advances　in　such　a　hydrogel,　a　polymer　network　of　strong　bonds　ruptures　at　the　front　of　the　crack　and　elicits　energy　dissipation　in　the　bulk　of　the　hydrogel.　The　constituent　that　dissipates　energy　in　the　bulk　of　the　hydrogel　is　called　a　toughener.　A　hypothesis　has　emerged　recently　that　tougheners　increase　fracture　toughness　greatly　but　contribute　little　to　fatigue　threshold.　Here　we　ascertain　this　hypothesis　by　studying　hydrogels　of　two　kinds,　identical　in　all　aspects　except　for　tougheners.　A　Ca-alginate/polyacrylamide　hydrogel　has　ionic　bonds,　which　act　as　tougheners,　resulting　in　a　toughness　of　3375　J/m(2)　and　a　threshold　of　35　J/m(2).　A　Na-alginate/polyacrylamide　hydrogel　has　no　ionic　bonds,　resulting　in　a　toughness　of　169　J/m(2)　and　a　threshold　of　17　J/m(2).　These　results　motivate　a　discussion　on　the　development　of　fatigue-resistant　hydrogels.