diff --git a/src/teaseisio.jl b/src/teaseisio.jl
index 5f5229f..022c2e0 100644
--- a/src/teaseisio.jl
+++ b/src/teaseisio.jl
@@ -883,7 +883,7 @@ function write_extentmanager(io::JSeis)
     write_parproperty(extentman, "VFIO_VERSION", "string", " 2006.2 ")
     write_parproperty(extentman, "VFIO_EXTSIZE", "long",   " $(io.trcextents[1].size) ")
     write_parproperty(extentman, "VFIO_MAXFILE", "int",    " $(length(io.trcextents)) ")
-    write_parproperty(extentman, "VFIO_MAXPOS",  "long",   " $(prod(io.axis_lengths[2:end]) * tracelength(io) - 1) ")
+    write_parproperty(extentman, "VFIO_MAXPOS",  "long",   " $(prod(io.axis_lengths[2:end]) * tracelength(io)) ")
     write_parproperty(extentman, "VFIO_EXTNAME", "string", " TraceFile ")
     write_parproperty(extentman, "VFIO_POLICY",  "string", " RANDOM ")
 
@@ -896,7 +896,7 @@ function write_extentmanager(io::JSeis)
     write_parproperty(extentman, "VFIO_VERSION", "string", " 2006.2 ")
     write_parproperty(extentman, "VFIO_EXTSIZE", "long",   " $(io.hdrextents[1].size) ")
     write_parproperty(extentman, "VFIO_MAXFILE", "int",    " $(length(io.hdrextents)) ")
-    write_parproperty(extentman, "VFIO_MAXPOS",  "long",   " $(prod(io.axis_lengths[2:end]) * headerlength(io) - 1) ")
+    write_parproperty(extentman, "VFIO_MAXPOS",  "long",   " $(prod(io.axis_lengths[2:end]) * headerlength(io)) ")
     write_parproperty(extentman, "VFIO_EXTNAME", "string", " TraceHeaders ")
     write_parproperty(extentman, "VFIO_POLICY",  "string", " RANDOM ")
 
diff --git a/test/test_teaseisio.jl b/test/test_teaseisio.jl
index fb6ca92..3a9f6c8 100644
--- a/test/test_teaseisio.jl
+++ b/test/test_teaseisio.jl
@@ -715,3 +715,21 @@ end
     write(io, trcs, :, :, :)
     @test isfile(joinpath(c,"second","foo","test.js","TraceFile0"))
 end
+
+# issue https://github.com/ChevronETC/TeaSeis.jl/issues/28
+@testset "teaseisio, VFIO_MAXPOS should be the size of the extent rather than an offset" begin
+    c = pwd()
+    ENV["JAVASEIS_DATA_HOME"] = joinpath(c, "primary")
+    io = jsopen(joinpath(ENV["JAVASEIS_DATA_HOME"], "test.js"), "w", axis_lengths=[3,3,10], secondaries=[joinpath(c,"second")], nextents=3)
+    close(io)
+    io = jsopen(joinpath(ENV["JAVASEIS_DATA_HOME"], "test.js"))
+
+    expected_extent_size = sizeof(Float32)*size(io,1)*size(io,2)*(div(10,3)+1)
+    expected_total_size = sizeof(Float32)*prod(size(io))
+    @test io.trcextents[1].size == expected_extent_size
+    @test io.trcextents[2].size == expected_extent_size
+
+    # the last extent size is computed from, in part, VFIO_MAXPOS
+    @test io.trcextents[3].size == expected_total_size - 2*expected_extent_size
+    rm(io)
+end